Efficient scaling of nonscalable MPEG-2 Video

ABSTRACT

To reduce bandwidth of non-scalable MPEG-2 coded video, certain non-zero AC DCT coefficients for the 8×8 blocks are removed from the MPEG-2 coded video. In one implementation, high-frequency AC DCT coefficients are removed at the end of the coefficient scan order. This method requires the least computation and is most desirable if the reduced-bandwidth video is to be spatially sub-sampled. In another implementation, the smallest-magnitude AC DCT coefficients are removed. This method may produce an undesirable increase in the frequency of occurrence of escape sequences in the (run, level) coding. This frequency can be reduced by retaining certain non-zero AC DCT coefficients that are not the largest magnitude coefficients, and by increasing a quantization scale to reduce the coefficient levels. The reduced-bandwidth video can be used for a variety of applications, such as browsing for search and play-list generation, bit stream scaling for splicing, and bit-rate adjustment for services with limited resources and for multiplexing of transport streams.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to processing of compressed visual data,and in particular the processing of compressed visual data in order toreduce data storage requirements or data transmission bandwidth at theexpense of decreased quality.

2. Background Art

It has become common practice to compress audio/visual data in order toreduce the capacity and bandwidth requirements for storage andtransmission. One of the most popular audio/video compression techniquesis MPEG. MPEG is an acronym for the Moving Picture Experts Group, whichwas set up by the International Standards Organization (ISO) to work oncompression. MPEG provides a number of different variations (MPEG-1,MPEG-2, etc.) to suit different bandwidth and quality constraints.MPEG-2, for example, is especially suited to the storage andtransmission of broadcast quality television programs.

For the video data, MPEG provides a high degree of compression (up to200:1) by encoding 8×8 blocks of pixels into a set of discrete cosinetransform (DCT) coefficients, quantizing and encoding the coefficients,and using motion compensation techniques to encode most video frames aspredictions from or between other frames. In particular, the encodedMPEG video stream is comprised of a series of groups of pictures (GOPs),and each GOP begins with an independently encoded (intra) I frame andmay include one or more following P frames and B frames. Each I framecan be decoded without information from any preceding and/or followingframe. Decoding of a P frame requires information from a preceding framein the GOP. Decoding of a B frame requires information from both apreceding and a following frame in the GOP. To minimize decoder bufferrequirements, transmission orders differ from presentation orders forsome frames, so that all the information of the other frames requiredfor decoding a B frame will arrive at the decoder before the B frame.

In addition to the motion compensation techniques for video compression,the MPEG standard provides a generic framework for combining one or moreelementary streams of digital video and audio, as well as system data,into single or multiple program transport streams (TS) which aresuitable for storage or transmission. The system data includesinformation about synchronization, random access, management of buffersto prevent overflow and underflow, and time stamps for video frames andaudio packetized elementary stream packets embedded in video and audioelementary streams as well as program description, conditional accessand network related information carried in other independent elementarystreams. The standard specifies the organization of the elementarystreams and the transport streams, and imposes constraints to enablesynchronized decoding from the audio and video decoding buffers undervarious conditions.

The MPEG-2 standard is documented in ISO/IEC International Standard (IS)13818-1, “Information Technology-Generic Coding of Moving Pictures andAssociated Audio Information: Systems,” ISO/IEC IS 13818-2, “InformationTechnology-Generic Coding of Moving Pictures and Associated AudioInformation: Video,” and ISO/IEC IS 13818-3, “InformationTechnology-Generic Coding of Moving Pictures and Associated AudioInformation: Audio,” which are incorporated herein by reference. Aconcise introduction to MPEG is given in “A guide to MPEG Fundamentalsand Protocol Analysis (Including DVB and ATSC),” Tektronix Inc., 1997,incorporated herein by reference.

MPEG-2 provides several optional techniques that allow video coding tobe performed in such a way that the coded MPEG-2 stream can be decodedat more than one quality simultaneously. In this context, the word“quality” refers collectively to features of a video signal such asspatial resolution, frame rate, and signal-to-noise ratio (SNR) withrespect to the original uncompressed video signal. These optionaltechniques are known as MPEG-2 scalability techniques. In the absence ofthe optional coding for such a scalability technique, the coded MPEG-2stream is said to be nonscalable. The MPEG-2 scalability techniques arevarieties of layered or hierarchical coding techniques, because thescalable coded MPEG-2 stream includes a base layer that can be decodedto provide low quality video, and one or more enhancement layers thatcan be decoded to provide additional information that can be used toenhance the quality of the video information decoded from the baselayer. Such a layered coding approach is an improvement over a simulcastapproach in which a coded bit stream for a low quality video istransmitted simultaneously with an independently coded bit stream forhigh quality video. The use of video information decoded from the baselayer for reconstructing the high quality video permits the scalablecoded MPEG-2 stream to have a reduced bit rate and data storagerequirement than a comparable simulcast data stream.

The MPEG-2 scalability techniques are useful for addressing a variety ofapplications, some of which do not need the high quality video that canbe decoded from a nonscalable coded MPEG stream. For example,applications such as video conferencing, video database browsing, andwindowed video on computer workstations do not need the high qualityprovided by a nonscalable coded MPEG-2 stream. For applications wherethe high quality video is not needed, the ability to receive, store, anddecode an MPEG-2 base-layer stream having a reduced bit rate or datastorage capacity may provide a more efficient bandwidth versus qualitytradeoff, and a more efficient complexity versus quality tradeoff. Ascalable coded MPEG-2 stream provides compatibility for a variety ofdecoders and services. For example, a reduced complexity decoder forstandard television could decode a scalable coded MPEG-2 stream producedfor high definition television. Moreover, the base layer can be codedfor enhanced error resilience and can provide video at reduced-qualitywhen the error rate is high enough to preclude decoding at high quality.

The MPEG scaling techniques are set out in sections 7.7 to 7.11 of theMPEG-2 standard video encoding chapter 13818-2. They are furtherexplained in Barry G. Haskell et al., Digital Video: An Introduction toMPEG-2, Chapter 9, entitled “MPEG-2 Scalability Techniques,” pp.183-229, Chapman & Hall, International Thomson Publishing, New York,1997, incorporated herein by reference. The MPEG scalability techniquesinclude four basic techniques, and a hybrid technique that combines atleast two of the four basic techniques. The four basic techniques arecalled data partitioning, signal-to-noise ratio (SNR) scalability,spatial scalability, and temporal scalability.

Data partitioning is a method of partitioning a single layer codedbit-stream into two classes, including a base layer “partition 0” and anenhancement layer “partition 1”. Partition 0 contains all high levelheader information as well as some low frequency discrete cosinetransform (DCT) coefficients. Partition 1 contains all remaining higherfrequency DCT coefficients and end-of-block (EOB) markers. Some syntaxelements belonging to partition 0 are redundantly copied to partition 1to facilitate error recovery. This duplicated information includes thesequence_header, GOP_header, picture_header, sequence_end_code,sequence_extension, picture_extension, and sequence_scalable_extension.This duplication ensures that there is proper synchronization andrecovery following a bit-stream error in the low priority enhancementlayer (partition 1) and introduces very little overhead. With respect tothe single layer coded bit-stream, the separation point between thesyntax elements to be included in the base and enhancement layers isindicated by a priority breakpoint (PBP) marker. The PBP can be adjustedat every picture slice. The PBP marker partitioning granularity is atthe (run, level) DCT event level of the coded block data. Datapartitioning is especially useful for error resilient video transmissionover asynchronous transfer mode (ATM) networks and other networks wheredata prioritization is possible. Data partitioning has a number ofshortcomings, including limited flexibility for PBP adjustment (in termsof partitioning granularity and update frequency), and the accumulationof drift errors over P pictures due to partially available coefficientinformation from a damaged enhancement layer.

SNR scalability is a method of generating a multiplex of bit-streamsrepresenting individual layers including a base layer which contains DCTcoefficients quantized at a basic moderate quality level, and one ormore SNR enhancement layers that contain DCT refinement coefficientsintended to enhance the precision of quantized DCT coefficientsreconstructed based on the content of all lower layers. Consequently,SNR scalability is also referred to as “Quantization Noise Scalability.”The layers in SNR scalability are all at the same spatial and temporalresolutions but cumulatively produce increasing quality levels startingwith the lowest quality at the base layer. The base layer includes allhigh level header information, all motion compensation and macroblock(MB) type information, and coarse quantized DCT coefficient information.The enhancement layers include quantized DCT refinement coefficientinformation, and some amount of overhead information. The slicestructure should be the same for all layers. Use of differentquantization matrices in the base and enhancement layers is allowed. Theoverhead required by SNR scalability results in a decreased bandwidthutilization efficiency compared to data partitioning. SNR scalability isespecially useful for simultaneous distribution of standard definitiontelevision and high-definition television, error-resilient videoservices over ATM and other networks, and multi-quality Video On Demand(VOD) services. SNR scalability has a number of shortcomings, includingincreased complexity and overhead as compared to data partitioning,inflexibility in bandwidth distribution among the layers primarily dueto the fact that all motion information has to be carried in the baselayer, and the shortcoming that no single SNR scalable codec caneliminate drift errors and also be reliable under lossy enhancementlayer transmission.

There are two variations to SNR scalability, namely, chroma simulcastand frequency domain SNR (FDSNR) scalability. Chroma simulcast providesa means for simultaneous distribution of video services that use 4:2:0and 4:2:2 chroma subsampling formats. The associated bit-streamstructure has three layers, including a base layer, an enhancementlayer, and a simulcast layer. The base layer is a distribution of videoin the 4:2:0 format. The enhancement layer provides SNR enhancement forthe luminance component of the base layer. The simulcast layer includeschrominance components of the 4:2:2 format.

Frequency domain SNR scalability provides a transform domain method toachieve spatial resolution scalability. The base layer is intended fordisplay at reduced spatial resolution and includes video encoded by aquantization matrix that allows a proper subset of normal size DCTtransform coefficients to be selected and included in the base layer foruse in conjunction with a smaller size DCT at the base layer decoder.The enhancement layer is the set of remaining normal size DCT transformcoefficients.

Spatial scalability provides an ability to decode video at differentspatial resolutions without first having to decode an entire (full-size)frame and then decimating it. The base layer carries the lowest spatialresolution version of the video obtained by decimating the original(full-size) video. Enhancement layers carry the differential informationrequired to generate successively higher spatial resolution versions ofthe video. Spatial scalability supports interoperability betweendifferent video resolution and formats, such as support for simultaneoustransmission of high definition television and standard definitiontelevision, and backward compatibility of MPEG-2 with differentstandards such as H.262 or MPEG-1. Spatial scalability supportserror-resilient video transmission on ATM and other networks. Decodercomplexity can scale with channel bandwidth. Spatial scalability has theadvantages of a high degree of flexibility in video resolution andformats to be used for each layer, and a high degree of flexibility inachieving bandwidth partitioning between layers. There are no decoderdrift problems because there are independent coding loops that are onlyloosely coupled. Spatial scalability, however, requires significantlyincreased complexity as compared to data partitioning and SNRscalability.

Temporal scalability provides an ability to decode video at differentframe rates without first having to decode every single frame. The baselayer carries the lowest frame rate version of the video coded by itselfat the basic temporal rate. This version of the video is obtained fromthe original full frame rate version by a temporal down-samplingoperation. The enhancement layers carry the information to construct theadditional frames required to generate successively higher temporalresolution versions of the video. Additional frames in each enhancementlayer are coded with temporal prediction relative to the frames carriedby lower layers. Temporal scalability provides simultaneous support fordifferent frame rates in the form of downward compatibility withlower-rate services, such as migration from first generation interlacedhigh definition television to high temporal resolution progressivehigh-definition television. Temporal scalability supportserror-resilient video transmission on ATM and other networks. Decodercomplexity can scale with channel bandwidth. Temporal scalability hasthe advantages of providing flexibility in achieving bandwidthpartitioning between layers. There are no decoder drift problems becausethere are independent coding loops that are only loosely coupled.Temporal scalability has less complexity and higher efficiency thanspatial scalability. Temporal scalability, however, provides a bandwidthpartitioning flexibility that is more limited than spatial scalabilitybecause temporal scalability uses the same spatial resolution in alllayers.

Hybrid scalability combines two scalabilities at a time from among SNR,spatial and temporal scalabilities. A base layer carries a basicquality, spatial and temporal resolution version of the intended videocontent. A first enhancement layer carries differential informationrequired to implement one of the two intended enhancements on the baselayer. A second enhancement layer carries differential informationrequired to implement the second intended enhancement on the combinationof the base and the first enhancement layers. Hybrid scalability isuseful in more demanding applications requiring scalability in two videoquality aspects within three or more bit-stream layers.

SUMMARY OF THE INVENTION

The basic objective of the present invention is to provide efficient SNRscaling of video originally present in a high-quality and nonscalableMPEG-2 transport stream. In other words, the present invention providesmethods for processing nonscalable MPEG-2 transport stream to produce alower bandwidth stream of video at lower quality levels. Software basedimplementations can operate in real-time upon the nonscalable MPEG-2transport stream to support a variety of applications, such as browsingand review of stored MPEG-2 clips for search and play-list generation,bit stream scaling for splicing, bit-rate adjustment via video qualityalteration for services with limited resources, and bit rate control ofmultiplexed transport streams.

In accordance with one aspect, the invention provides a method ofscaling non-scalable MPEG-2 coded video to produce reduced-bandwidth,reduced-quality MPEG-2 coded video. The non-scalable MPEG-2 coded videoincludes a set of non-zero AC discrete cosine transform (DCT)coefficients for 8×8 blocks of the non-scalable MPEG-2 coded video. Themethod includes removing non-zero AC DCT coefficients from thenon-scalable MPEG-2 coded video so that there are no more than aselected number of non-zero AC DCT coefficients in the reduced-qualityMPEG-2 coded video for each of the 8×8 blocks.

In accordance with another aspect, the invention provides a method ofscaling non-scalable MPEG-2 coded video to produce reduced-bandwidth,reduced-quality MPEG-2 coded video. The non-scalable MPEG-2 coded videoincludes a set of non-zero AC discrete cosine transform (DCT)coefficients for 8×8 blocks of the non-scalable MPEG-2 coded video. Themethod includes removing non-zero AC DCT coefficients from thenon-scalable MPEG-2 coded video by selecting an initial set of thenon-zero AC DCT coefficients to include in the reduced-quality MPEG-2coded video, (run, level) coding the initial set of the non-zero AC DCTcoefficients, and upon finding an escape sequence in the (run, level)coding of the initial set of the non-zero AC DCT coefficients, searchingfor a non-zero AC DCT coefficient of the non-scalable MPEG-2 coded videothat is not in the initial set of the non-zero AC DCT coefficients andthat would eliminate the escape sequence when included in thereduced-quality MPEG-2 coded video, and upon finding such a non-zero ACDCT coefficient of the non-scalable MPEG-2 coded video, including such anon-zero AC DCT coefficient in the reduced-quality MPEG coded video.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description with reference to theaccompanying drawings, in which:

FIG. 1 is a block diagram of a data network including a video fileserver implementing various aspects of the present invention;

FIG. 2 is a flowchart of a procedure executed by a stream servercomputer in the video file server of FIG. 1 to service client requests;

FIG. 3 is a flowchart of a procedure for splicing MPEG clips;

FIG. 4 is a flowchart of a procedure for seamless video splicing of MPEGclips;

FIG. 5 is a more detailed flowchart of the procedure for seamless videosplicing of MPEG clips;

FIG. 6 is a continuation of the flowchart begun in FIG. 5;

FIG. 7 is a timing diagram showing a timing relationship between videopresentation units (VPUs) and associated audio presentation units (APUs)in an original MPEG-2 coded data stream;

FIG. 8 is a timing diagram showing a timing relationship between videopresentation units (VPUs) and associated audio presentation units (APUs)for a fast-forward trick-mode stream;

FIG. 9 is a flowchart of a procedure for selection and alignment ofaudio presentation units (APUs) in the fast-forward trick-mode stream;

FIG. 10 is a flowchart of a procedure for producing a trick-mode MPEG-2transport stream from a regular MPEG-2 transport stream (TS);

FIG. 11 is a diagram illustrating relationships between the MPEGdiscrete cosine transform (DCT) coefficients, spatial frequency, and thetypical zig-zag scan order;

FIG. 12 is a diagram illustrating a relationship between an MPEG-2 codedbit stream and a reduced-quality MPEG-2 coded bit stream resulting fromtruncation of high-order DCT coefficients;

FIG. 13 is a flowchart of a procedure for scaling MPEG-2 coded videousing a variety of techniques;

FIG. 14 is a flowchart of a procedure for signal-to-noise ratio scalingMPEG-2 coded video using a frequency-domain low-pass truncation(FDSNR_LP) technique;

FIG. 15 is a flowchart of a procedure for signal-to-noise ratio scalingMPEG-2 coded video using a frequency-domain largest-magnitudecoefficient selection (FDSNR_LM) technique;

FIG. 16 is a flowchart of a procedure that selects one of a number oftechniques for finding a certain number “k” of largest values out of aset of “n” values;

FIG. 17 is a flowchart of a procedure for finding a certain number “k”of largest values from a set of “n” values, which is used in theprocedure of FIG. 16 for the case of k<<½ n;

FIG. 18 is a diagram of a hash table and associated hash lists;

FIG. 19 is a flowchart of a procedure for finding a certain number “k”of values that are not less than the smallest of the “k” largest valuesin a set of “n” values beyond a certain amount.

FIG. 20 is a flowchart of modification of the procedure of FIG. 15 inorder to possibly eliminate escape sequences in the (run, level) codingof the largest magnitude coefficients;

FIG. 21 is a flowchart of a subroutine called in the flowchart of FIG.20 in order to possibly eliminate an escape sequence;

FIG. 22 is a first portion of a flowchart of a procedure for scaling anMPEG-2 coded video data stream using the modified procedure of FIG. 20while adjusting the parameter “k” to achieve a desired bit rate, andadjusting a quantization scaling factor (QSF) to achieve a desiredfrequency of occurrence of escape sequences;

FIG. 23 is a second portion of the flowchart begun in FIG. 22;

FIG. 24 is a simplified block diagram of a volume containing a mainfile, a corresponding fast forward file for trick mode operation, and acorresponding fast reverse file for trick mode operation;

FIG. 25 is a more detailed block diagram of the volume introduced inFIG. 24;

FIG. 26A is a diagram showing video file access during a sequence ofvideo operations including transitions between the main file, therelated fast forward file, and the related fast reverse file;

FIG. 26B shows a script of a video command sequence producing thesequence of video play shown in FIG. 26A;

FIG. 27 is a table of read and write access operations upon the volumeof FIG. 24 and access modes that are used for the read and write accessoperations;

FIG. 28 is a hierarchy of video service classes associated with the fastforward file and the fast reverse file in the volume of FIG. 25; and

FIG. 29 shows a system for modifying and combining an MPEG-2audio-visual transport stream with an MPEG-2 closed-captioning transportstream to produce a multiplexed MPEG-2 transport stream having the samebit rate as the original MPEG-2 audio-visual transport stream.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and will be described in detail. It should beunderstood, however, that it is not intended to limit the form of theinvention to the particular forms shown, but on the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

With reference to FIG. 1, there is shown a block diagram of a datanetwork 20 linking a number of clients 21, 22, 23 to a video file server24 implementing various aspects of the present invention. The video fileserver 24 includes at least one stream server computer 25 and a datastorage system 26. The stream server computer 25 has a processor 27 anda network link adapter 28 interfacing the processor to the data network20. The processor 27 executes a data streaming program 29 in memory 30in order to stream MPEG coded video in real-time to the clients.

Client requests for real-time video are placed in client play lists 31in order to schedule in advance video file server resources for thereal-time streaming of the MPEG coded video. The play lists 31 specify asequence of video clips, which are segments of MPEG-2 files 32, 33 indata storage 34 of the data storage system 26. The stream serverprocessor 27 accesses a client play list in advance of the time to beginstreaming MPEG coded video from a clip, and sends a video prefetchcommand to a storage controller 35 in the data storage system 26. Thestorage controller responds to the video prefetch command by accessingthe clip in the data storage 34 to transfer a segment of the clip tocache memory 36. When the video data of the segment needs to be sent tothe client, the stream server processor 27 requests the data from thestorage controller 35, and the storage controller immediately providesthe video data from the cache memory 36. Further details regarding apreferred construction and programming of the video file server 24 aredisclosed in Duso et al., U.S. Pat. No. 5,892,915 issued Apr. 6, 1999,entitled “System Having Client Sending Edit Commands to Server DuringTransmission Of Continuous Media From One Clip in Play List for Editingthe Play List,” incorporated herein by reference.

In accordance with an aspect of the invention, the stream servercomputer 25 executes an MPEG scaling program 38 to producereduced-quality MPEG coded video from nonscalable MPEG-2 coded video bytruncating discrete cosine transform (DCT) AC coefficients from thecoded blocks in the MPEG-2 coded video data. The reduced-quality MPEGcoded video can be produced during ingestion of an MPEG-2 file 32 fromthe network 20, and stored in one or more associated files 37.Alternatively, the reduced-quality MPEG coded video in the files 37could be produced as a background task from the MPEG-2 file 32.Reduced-quality MPEG coded video could also be produced in real-timefrom an MPEG-2 file 33 during streaming of the reduced-quality MPEGcoded video from the stream server computer 25 to the network 20. Thereduced-quality MPEG coded video is useful for a variety ofapplications, such as browsing and review of stored MPEG-2 assets forsearch and play-list generation, bit stream scaling for splicing, andbit-rate adjustment via video quality alteration for services withlimited resources.

A typical example of browsing for play-list generation involvessearching stored assets in a multi-media data base for segments of adesired content to be included in the play list, and in particularselecting the beginning frame and ending frame of each segment to beincluded. Such editing occurs often in the broadcast environment forinserting commercials and news clips into pre-recorded televisionprogramming, and for editing movies for content and time compression.The decoding technique of the present invention permits a PC workstation23 to perform the decoding and display in real-time by execution of asoftware program. An operator can view the video content in a displaywindow 39 in a fast-forward or fast-reverse mode, stop at and resumefrom freeze frames that are valid “in points” and “out points” forseamless splicing, and select an in-point and out-point for a nextsegment to be included in the play list. The stream server computer 25could also include a seamless splicing program 40 providing seamlesstransitions between video segments that are contiguous in a play listand are from different video clips.

For seamless splicing, it is often necessary to reduce the bitrate forone or more frames at the end of a first segment prior to splicing to asecond segment. In this case the bitrate must be reduced to avoid bufferoverflow as a result of displaying the original frames at the end of thefirst segment. One method of reducing the bitrate is to insert a freezeframe at the end of the first segment, but this has the disadvantage ofintroducing distortion in the temporal presentation of the frames andprecluding frame accuracy. A less disruptive method is to use thepresent invention for reducing the bitrate for a lower-qualitypresentation of one or more frames at the end of the first segment.

The present invention can also reduce the bit transmission rate andstorage requirements for MPEG-2 applications by altering the videoquality. For example, different clients may present different bandwidthaccess requests for video from nonscalable MPEG-2 files 32, 33 in thevideo file server. Also, temporary network congestion may limit thebandwidth available to satisfy a request for real-time streaming ofvideo data. In each case, the present invention can alter the videoquality to meet the desired or available bandwidth to satisfy therequest.

With reference to FIG. 2, there is shown a flowchart of a procedureexecuted by a stream server computer in the video file server of FIG. 1to service client requests. In a first step 50, execution branches tostep 51 when a client request is not a request for real-time streaming.If the request is a request to input a new MPEG-2 file, then executionbranches to step 52 to input the new MPEG-2 file and to create areduced-quality version of the MPEG-2 file as available resourcespermit. If the request is not a request to input a new MPEG-2 file, thenexecution continues from step 51 to step 53. In step 53, executionbranches to step 54 if the request is for play list editing. In step 54,the client may browse through the reduced-quality MPEG file to selectin-points and out-points of clips to be spliced.

In step 50, when the request is for real-time streaming, then executionbranches to step 55. In step 55, if there is network congestion so thatthere is insufficient bandwidth to transmit a stream of original-qualityMPEG-2 coded video, then execution branches to step 56 to streamcompressed video from the reduced-quality MPEG file. If noreduced-quality MPEG file is available for the desired clip, then thereduced-quality MPEG coded video to be streamed is produced in real-timefrom the original-quality MPEG-2 coded video. There are alsoapplications, such as the display of spatially down-sampled video in asmall display window (39 in FIG. 1), for which the client may requestreduced-quality MPEG coded video. In this case, in the absence ofnetwork congestion, execution will continue from step 55 to step 57, andbranch from step 57 to step 56 for streaming of reduced-quality MPEGcoded video to the client.

Reduced-quality MPEG coded video is also useful for “trick-mode”operation. Trick-mode refers to fast forward or fast reverse display ofvideo, in a fashion analogous to the fast forward and fast reverseplayback functions of a video cassette recorder (VCR). The problem withtrick-mode operation is that the speed of the MPEG stream cannot simplybe speeded up because the transmission bandwidth would be excessive anda conventional MPEG-2 decoder will not be able to handle the increaseddata rate or even if the decoder would have been able to support theincreased data rate, such a change in the original operating conditionsis not allowable. For this reason, in trick-mode, neither the originaldisplay rate of 29.97 frames per second (for NTSC or 25 frames persecond for PAL) nor the original transport stream (TS) multiplex rateshould change. Nor is it possible to simply decimate frames since onlythe I frames are independently coded, and the P frames and B frames needthe content of certain other frames for proper decoding. The I framestypically occur once for every 15 frames. Assuming that this conventionis followed in the encoding process, it would be possible to preserveand play each I frame from each and every group of pictures (GOP),resulting in a 15 times slower temporal sampling rate, or a 1 to 15speeding up of motion if the I frames only are played back at thenominal NTSC rate of approximately 30 frames per second. Consequently,the content of a 60 minutes duration clip will be covered in 4 minutes.Unfortunately the average information content per frame for the I framesis more than four times the average information content of the P and Bframes. Therefore, the trick-mode cannot be implemented simply bytransmitting only the I frames for a speed-up by a factor of 15, becausethis would need an increase in the TS multiplex rate over the nominalrate.

In particular, the average information content of an I frame has beenmeasured to be about 56374.6 bytes. If the I frames only are transmittedat the standard NTSC rate, then the bit transmission rate would be:8(bits per byte)*56,374.6(bytes per frame)*29.97(frames per sec.) orabout 13,516,374.1 bits per second only for the video stream, which issignificantly above—almost 3.38 times—the original rate of 4 megabitsper second used in this test. This calculation, being based on anaverage quantity, is ignoring the indispensable need for an actuallyhigher transport rate to provide some safety margin to handleshort-term-sustained large size I frame chains (bursts) whichpractically always happen. Clearly, some form of modification in thetrick-mode operation definition is required to handle this problem andpull the bit-rate requirement down to the nominal 4 megabits per second.

Two degrees of freedom are available to achieve such a reduction in therequired bit-rate for trick-mode operation. The first is I framecompression quality and the second is a motion speed-up ratio. Withrespect to compression quality, it is well known that human observers'perception of image detail degrades with increasing motion speed ofobjects in the scene. Based on this fact, the type of D pictures wereintroduced in MPEG-1 video syntax for fast visible (forward or reverse)search purposes. (See ISO/IEC 11172-2: 1993 InformationTechnology—Coding of moving pictures and associated audio for digitalstorage media at up to about 1.5 Mbits/s—Part 2: Video, Annex D.6.6.Coding D-Pictures, p. 102). D pictures make use of only the DCcoefficients in intra coding to produce very low quality (in terms ofSNR) reproductions of desired frames which were judged to be of adequatequality in fast search mode.

In order to provide support for enhanced quality trick-mode operation,the quality of the original I frames can be reduced by the preservationof just a sufficient number of AC DCT coefficients to meet the bit-ratelimitation. Based on experiments with two standard video test sequences(one encoded at 15 Mbits/sec. and the other at 24 Mbits/sec. and bothwith I frames only), it is observed that the bandwidth for I frames canbe scaled to one half by keeping about 9 lowest order AC coefficientsand eliminating the rest. This scheme provides good quality even at thefull spatial and temporal resolution, much better than D pictures.

The inherent speed-up ratio lower bound imposed by the GOP structure canbe relaxed and further lowered by freeze (P) frame substitution inbetween genuine (SNR scaled or non-scaled) I frames. The maximum numberof freeze frames that can be inserted before visually disturbing motionjerkiness occurs, is very likely to depend heavily on the original GOPstructure (equivalently the separation between I frames of the originalsequence) and the original amount of motion in the clip. However, 1, 2or 3 freeze frame substitutions in between genuine I frames presentreasonable choices which will yield speed-up ratios of 1 to 7.5, 1 to 5and 1 to 3.75 respectively instead of the 1 to 15 speed-up ratioprovided by the genuine I frames only implementation. (These ratios arecomputed by a first-order approximation that neglects a slight increasein bandwidth required by the consecutive freeze frames, which areinserted in between genuine I frames and can typically be made verysmall in size in comparison to the average size of a genuine I frame.Therefore, the insertion of 1, 2, 3 freeze frames will result inbandwidth reductions of 2 to 1, 3 to 1 and 4 to 1 respectively. Theaccuracy of this approximation degrades as more consecutive freezeframes and/or SNR scaling is employed.) An easy way to see the validityof these approximate figures is to note for example that in the case ofI freeze frame insertion, the total presentation time of the trick-modeclip for an originally 60 minutes duration asset will increase from 4minutes to 8 minutes. Since due to the underlying assumption of thefirst-order approximation stated above, the same amount of data (Iframes only) will be transmitted in this doubled time interval, thebandwidth requirement will be halved. The final choice for trick-modeimplementation should reflect a balanced trade-off along these twodegrees of freedom. For example, SNR scaling of I frames down to 9 ACcoefficients can be used along with single freeze frame insertionbetween I frames. These two choices, both of which are individuallycapable of providing a 2 to 1 bandwidth reduction as discussed before,will yield a combined 4 to 1 bandwidth reduction which will comfortablybring the non-scaled I frame-only bit-rate of 13516374.1 bits/sec. downto below the 4 Mbits/sec. quota. If the visual quality provided by 9 ACcoefficients is not considered adequate, then SNR scaling could be tunedto keep more AC coefficients at the expense of a smaller bandwidthreduction. This, however, could be compensated consequently byincreasing the number of freeze frames to be used in between I frames.Coarser quantization (and therefore poorer visual quality) can betolerated at high trick-mode speeds and better visual quality should beretained at lower trick-mode speeds.

With reference to FIG. 2, if the client has requested trick-modeoperation, execution branches from step 58 to step 59. In step 59,execution branches to step 60 for a low value of speed-up. In step 60,the trick-mode stream is produced by streaming original-quality I framesand inserting three freeze frames per I frame, to yield a speed-upfactor of 15/4=3.75 based on an original MPEG-2 coded stream having oneI frame for every 15 frames. For a higher speed-up factor, executionbranches from step 59 to step 61. In step 61, either one or two freezeframes are selected per I frame to provide a speed-up factor of15/2=7.5, or 15/3=5 respectively. Then in step 62 the trick-mode streamis produced by streaming reduced-quality I frames and inserting theselected number of freeze frames between the reduced-quality I frames.If a trick-mode operation is not requested in step 58, then executioncontinues from step 58 to step 63. In step 63, the stream servercomputer streams original-quality MPEG-2 coded data to the client.Further details regarding trick-mode operation are described below withreference to FIGS. 7 to 10.

FIGS. 3 to 6 show further details regarding use of the present inventionfor MPEG splicing. In particular, reduced-quality frames are substitutedfor the freeze frames used in the seamless splicing procedure found inthe common disclosure of Peter Bixby et al., U.S. application Ser. No.09/539,747 filed Mar. 31, 2000; Daniel Gardere et al., U.S. applicationSer. No. 09/540,347 filed Mar. 31, 2000; and John Forecast et al. U.S.application Ser. No. 09/540,306 filed Mar. 31, 2000; which are allincorporated by reference herein. The common disclosure in these U.S.applications considered pertinent to the present invention is includedin the written description below with reference to FIGS. 3 to 6 in thepresent application (which correspond to FIGS. 19, 22, 23, and 24 ineach of the cited U.S. applications).

FIG. 3 shows a basic procedure for MPEG splicing. In the first step 121,the splicing procedure receives an indication of a desired end frame ofthe first clip and a desired start frame of the second clip. Next, instep 122, the splicing procedure finds the closest I frame preceding thedesired start frame to be the In Point for splicing. In step 123, thesplicing procedure adjusts content of the first clip near the end frameof the first clip and adjusts content of the second clip near the inpoint in order to reduce presentation discontinuity (due to decoderbuffer underflow) and also to prevent decoder buffer overflow whendecoding the spliced MPEG stream. Finally, in step 124, theconcatenation of the first clip up to about the Out Point and the secondclip subsequent to about the In Point is re-formatted, includingre-stamping of the presentation time stamps (PTS), decoding time stamps(DTS), and program clock reference (PCR) values for the audio and videostreams in the second clip.

Considering now video splicing, the splicing procedure should ensure theabsence of objectionable video artifacts, preserve the duration of thespliced stream, and if possible, keep all of the desired frames in thespliced stream. The duration of the spliced stream should be preservedin order to prevent any time drift in the scheduled play-list. In somecases, it is not possible to keep all of the original video frames dueto buffer problems.

Management of the video buffer is an important consideration in ensuringthe absence of objectionable video artifacts. In a constant bit rate(CBR) and uniform picture quality sequence, subsequent picturestypically have coded representations of drastically different sizes. Theencoder must manage the decoder's buffer within several constraints. Thebuffer should be assumed to have a certain size defined in the MPEG-2standard. The decoder buffer should neither overflow nor underflow.Furthermore, the decoder cannot decode a picture before it receives itin full (i.e. completely). Moreover, the decoder should not be made to“wait” for the next picture to decode; this means that every 40 ms inPAL and {fraction (1/29.97)} second in NTSC, the decoder must haveaccess to a full picture ready to be decoded.

The MPEG encoder manages the video decoder buffer through decode timestamps (DTS), presentation time stamps (PTS), and program clockreference (PCR) values. When splicing the end of a first clip to thebeginning of a second clip, there will be a problem of video buffermanagement if a duration of time DTS_(L1)−T_(e) is different from aduration of time DTS_(F2)−PCR_(e2) minus one video frame (presentation)interval, where DTS_(L1) is the DTS at the end of the first clip andindicates the time at which the video decoder buffer is emptied of videodata from the first clip, T_(e) is the time at which the last videoframe's data is finished being loaded into the video decoder buffer,DTS_(F2) is the DTS of the first frame of the second clip, and PCR_(e2)is the PCR of the second clip extrapolated from the value of the mostrecent received genuine PCR record, to the first byte of the pictureheader sync word of the first video frame in the clip to start. Theextrapolation adjusts this most recently received genuine PCR recordvalue by the quotient of the displacement in data bits of the clip fromthe position where it appears in the second clip to the position atwhich video data of the first frame of the second clip begins, dividedby the data transmission bit rate for transmission of the clip to thedecoder. Because the time PCR_(e2) must immediately follow T_(e), therewill be a gap in the decoding and presentation of video frames ifDTS_(F2)−PCR_(e2) is substantially greater than DTS_(L1)−T_(e) plus onevideo frame interval. In this case, the buffer will not be properly fullto begin decoding of the second clip one video frame interval after thelast frame of the first clip has been decoded. Consequently, either thesecond clip will be prematurely started to be decoded or the decoderwill be forced to repeat a frame one or more times after the end of thedisplay of the last frame from the first clip to provide the requireddelay for the second clip's buffer build-up. In the case of a prematurestart for decoding the second clip, a video buffer underflow risk isgenerated. On the other hand, in case of repeated frames, the desiredframe accuracy for scheduled play-lists is lost besides the fact thatneither a precise timing adjustment can be achieved through thisprocedure.

If DTS_(F2)−PCR_(e2) is substantially less than DTS_(L1)−T_(e) plus onevideo frame interval, then the decoder will not be able to decode thefirst frame of the second clip at the specified time DTS_(F2) becausethe last frame of the first clip will not yet have been removed from thevideo buffer. In this case a video buffer overflow risk is generated.Video buffer overflow may present a problem not only at the beginning ofthe second clip, but also at a subsequent location of the second clip.If the second clip is encoded by an MPEG-2 compliant encoder, then videobuffer underflow or buffer overflow will not occur at any time duringthe decoding of the clip. However, this guarantee is no longer valid ifthe DTS_(F2)−PCR_(e2) relationship at the beginning of the second clipis altered. Consequently, to avoid buffer problems, the buffer occupancyat the end of the first clip must be modified in some fashion. Thisproblem is inevitable when splicing between clips having significantlydifferent ending and starting buffer levels. This is why the Society ofMotion Picture and Television Engineers (SMPTE) has defined some splicetypes corresponding to well-defined buffer levels. (See SMPTE Standard312M, entitled “Splice Points for MPEG-2 Transport Streams,” SMPTEJournal, Nov. 1998.) In order to seamlessly splice the first clip to thesecond clip, the content of the first clip (towards its end) is modifiedso that PCR_(e2) can immediately follow T_(e) (by one byte transmissiontime) and DTS_(F2) can just follow DTS_(L1) (by one video framepresentation interval).

FIG. 4 shows a flow chart of a seamless video splicing procedure thatattains the desired condition just described above. In a first step 141,the first DTS of the second clip is anchored at one frame interval laterthan the last DTS of the first clip in order to prevent a video decodingdiscontinuity. Then, in step 142, the procedure branches depending onwhether the PCR extrapolated to the beginning frame of the second clipfalls just after the ending time of the first clip. If so, then thesplice will be seamless with respect to the original video content.Otherwise, the procedure branches to step 143. In step 143, the contentof the first clip is adjusted so that the PCR extrapolated to thebeginning frame of the second clip falls just after the ending time ofthe first clip. Therefore the desired conditions for seamless videosplicing are achieved.

With reference to FIG. 5, there is shown a more detailed flow chart of aseamless video splicing procedure. In a first step 151, the procedureinspects the content of the s first clip to determine the last DTS/PTSof the first clip. This last DTS/PTS of the first clip is designatedDTS_(L1). Next, in step 152, the procedure inspects the content of thefirst clip to determine the time of arrival (T_(e)) of the last byte ofthe first clip. In step 153, the procedure adds one frame interval toDTS_(L1) to find the desired first DTS location for the second clip. Thesum, designated DTS_(F1), is equal to DTS_(L1)+1/FR, where FR is thevideo frame rate. In step 154, while keeping the DTS-PCR_(e)relationship unaltered for the second clip, the procedure finds the timeinstant, designated T_(S), at which the first byte of the second clipshould arrive at the decoder buffer. This is done by calculating

T _(START) =DTS _(F2) −PCR _(e2), and T _(S) =DTS _(F1) −T _(START).

Continuing in FIG. 6, in step 155, execution branches depending onwhether T_(S) is equal to T_(e) plus 8 divided by the bit rate. If not,then the clips to be spliced need modification before concatenation, andexecution branches to step 156. In step 156, execution branchesdepending on whether T_(S) is less than T_(e) plus 8 divided by the bitrate. If not, then there is an undesired gap in between the clips to bespliced, and execution branches to step 157. In step 157, null packetsare inserted into the clips to be spliced to compensate for the gap. Thegap to be compensated has a number of bytes, designated G_(r), equal to(T_(S)−T_(e))(BIT RATE)/8 minus one. If in step 156, T_(S) is less thanT_(e) plus 8 divided by the bit rate, then execution continues from step156 to step 158 to open up a certain amount of space in the first clipto achieve T_(S)=T_(e)+8/(BIT RATE). The number of bytes to drop is oneplus (T_(e)−T_(S))(BIT RATE)/8. If possible, the bytes are dropped byremoving null packets. Otherwise, one or more frames at the end of thefirst clip are replaced with corresponding reduced-quality frames, whichhave fewer bytes than the original-quality frames at the end of thefirst clip.

If in step 155 T_(S) is found to be equal to T_(e) plus 8 divided by thebit rate, then execution continues to step 159. Execution also continuesto step 159 from steps 157 and 158. In step 159, the transport streamsfrom the two clips are concatenated. Finally, in step 160, a subroutineis called to compute a video time stamp offset, designated asV_(OFFSET). This subroutine finds the DTS of the last video frame (indecode order) of the first clip. This DTS of the last video frame of thefirst clip is denoted DTS_(VL1). Then the subroutine finds the originalDTS of the first frame to be decoded in the second clip. This DTS of thefirst frame to be decoded in the second clip is denoted DTSV_(F2).Finally, the subroutine computes the video time stamp offset V_(OFFSET)as DTS_(VL1)−DTS_(VF2) plus one video frame duration.

FIGS. 7 to 10 show further details regarding trick-mode operation. FIG.7 shows a timing relationship between video presentation units (VPUs)and associated audio presentation units (APUs) in an original MPEG-2coded data stream, and FIG. 8 shows similar timing for the fast-forwardtrick-mode stream produced from the original data stream of FIG. 7. (Thefast-forward trick-mode stream is an example of a trick-mode stream thatcould be produced in step 60 of FIG. 2.) The original data stream hassuccessive video presentation units for video frames of type I, B, B, P,B respectively. The trick-mode stream has successive video presentationunits for video frames of types I, F, F, I, F where “F” denotes a freezeP (or possibly B) frame. Each I frame and immediately following F framesproduce the same video presentation units as a respective I frame in theoriginal data stream of FIG. 7, and in this example, one in every framesin the original data stream is an I frame. Each freeze frame is coded,for example, as a P frame repeating the previous I frame or the previousP-type freeze-frame (in display order). In each freeze frame, the frameis coded as a series of maximum-size slices of macroblocks, with aninitial command in each slice indicating that the first macroblock is anexact copy of the corresponding macroblock in the previous frame(achieved by predictive encoding with a zero valued forward motioncompensation vector and no encoded prediction error), and two consequentcommands indicating that the following macroblocks in the slice untiland including the last macroblock of the slice are all coded in the sameway as the first macroblock.

For trick-mode operation, there is also a problem of how to select audiopresentation units (APU) to accompany the video presentation units thatare preserved in the trick-mode stream. Because the video presentationunits (VPU) have a duration of ({fraction (1/29.97)}) sec. or about33.37 msec. and the audio presentation units (APU) have a duration of 24msec., there is neither a one-to-one correspondence nor alignmentbetween VPUs and APUs. In a preferred implementation, the audio contentof a trick-mode clip is constructed as follows. Given the totalpresentation duration ({fraction (1/29.97)}) sec. or about 33.37 msec.for a single video frame, it is clear that always at least one and atmost two 24 msec. long audio presentation units (APU) will start beingpresented during the end-to-end presentation interval of each videoframe. This statement refers to the original clip and does not considerany audio presentation unit whose presentation is possibly continuing asthe video frame under consideration is just put on the display. Thefirst of the above mentioned possibly two audio presentation units willbe referred to as the aligned audio presentation unit with respect tothe video frame under consideration. For example, in FIG. 8, the APU_(j)is the aligned audio presentation unit with respect to the VPU_(i). Now,when the I frames are extracted and possibly SNR scaled and possiblyfurther interleaved with a number of freeze P frames in between them toproduce the trick-mode video packetized elementary stream (PES), theassociated trick-mode audio stream is constructed as follows. For each Itype video frame presentation interval (and for that matter also forfreeze P type video frames) in this trick-mode clip, the above statedfact of at least one (and at most two) audio presentation unit beingstarted, holds. Then for each I frame presentation interval in thetrick-mode clip, once any possibly previously started and continuingaudio presentation unit ends, insert its aligned audio presentation unit(from the original clip) and continue inserting APUs from the originalclip subsequent to the aligned one until covering the rest of the Iframe presentation interval and also any possibly following freeze Pframe presentation intervals until crossing into and overlapping (orless likely aligning) with the next I frame's presentation interval. InFIG. 8, for example, the audio presentation units APU_(j), APU_(j+1),APU_(j+2), and APU_(j+3) are inserted, until crossing into andoverlapping with the next I frame VPU_(i+15). Following APU_(j+3) isinserted APUk, which designates the APU aligned with VPU_(i+15) in theoriginal stream. Clearly, the final alignment of (the aligned andconsequent) audio presentation units with respect to their associated Iframes will be slightly different in the trick-mode clip as compared tothe original clip. However, considering how the trick-mode audiocomponent will sound like, this poses no problem at all.

FIG. 9 is a flowchart of a procedure for producing the desiredsequencing of audio presentation units (APUs) in the fast-forwardtrick-mode stream. This procedure scans the audio elementary stream inthe original MPEG-2 stream to determine the sequence of APUs in theoriginal stream and their presentation-time alignment with the I framesin the video elementary stream of the original MPEG-2 transport stream,while selecting APUs to include in the trick-mode stream. In a firststep 171, execution proceeds once the end of the current APU is reached.If the end of the current APU has not entered a new VPU (i.e., thebeginning of the current APU is within the presentation time of one VPUand the end of the current APU is within the presentation time of thesame VPU), or if it has entered a new VPU (i.e., the beginning of thecurrent APU is within the presentation time of one VPU and the end ofthe current APU is within the presentation time of a new (next) VPU) butthe new VPU is not an I frame, then execution branches to step 174. Instep 174, an APU pointer is incremented, and in step 175 executionproceeds into this next APU. If in step 173 the end of the current APUextends into an I frame, then in step 176 the APU pointer is advanced topoint to the first APU beginning within the duration of the VPU of the Iframe in the original MPEG-2 stream.

FIG. 10 is a flowchart of a procedure for producing a trick-mode streamfrom an MPEG-2 transport stream (TS). In a first step 181, the MPEG-2 TSis inputted. In step 182, the video elementary stream (VES) is extractedfrom the TS. In step 183, a concurrent task extracts the audioelementary stream (AES) from the TS. In step 184, I frames are extractedfrom the VES and valid packetized elementary stream (PES) packets areformed encapsulating the I frames. In step 185, the I frames are SNRscaled, for the high speed cases of the trick-mode. In step 186, P-typefreeze frames are inserted into the stream of SNR scaled I frames (inbetween the scaled I frames), and valid PES packets are formed for thetrick-mode VES encapsulating the P-type freeze frames and the SNR scaledI frames. Concurrently, in step 187, appropriate audio access units(from the originally input MPEG-2 TS asset) are selected andconcatenated based on the structure of the VES being formed for thetrick-mode clip, as described above with reference to FIG. 9, and validPES packet encapsulation is formed around these audio access units.Finally, in step 188, the trick-mode TS stream is generated bymultiplexing the trick-mode VES from step 186 into a system information(SI) and audio PES carrying TS skeleton including the audio PES packetsfrom step 187.

FIGS. 11 to 19 include details of the preferred techniques fortruncating AC DCT coefficients for producing low-quality MPEG codedvideo from original-quality MPEG-2 coded video. Most of these techniquesexploit the fact that in the typical (default) zig-zag scan order, thebasis functions for the high-order AC DCT coefficients have anincreasing frequency content. FIG. 11, for example, shows a matrix ofthe DCT coefficients C_(ij). The row index (i) increases with increasingvertical spatial frequency in a corresponding 8×8 coefficient block, andthe column index (j) increases with increasing horizontal spatialfrequency in the corresponding 8×8 coefficient block. The coefficientC₁₁ has zero frequency associated with it in both vertical andhorizontal directions, and therefore it is referred to as the DCcoefficient of the block. The other coefficients have non-zero spatialfrequencies associated with their respective basis functions, andtherefore they are referred to as AC coefficients. Each coefficient hasan associated basis function f_(ij)(x,y) that is separable into x and ycomponents such that f_(ij)(x,y)=f_(i)(y)f_(j)(x). The x and y componentfunctions f_(i)(y) and f_(j)(x) are shown graphically in FIG. 11 ascosine functions in order to illustrate their associated spatialfrequencies. In practice, the component functions are evaluated atdiscrete points for the 64 pixel positions in the 8×8 blocks, so thateach of the DCT basis functions is an 8×8 array of real numbers. Inparticular, the component functions are:

f _(i)(y)=SQRT((2−δ_(i−1))/8)(cos((π/8)(y−1/2)(i−1))) for y=1, 2, 3, . .. , 8

f _(j)(y)=SQRT((2−δ_(i−1))/8)(cos((π/8)(x−1/2)(j−1))) for x=1, 2, 3, . .. , 8

The heavy black line through the matrix of coefficients in FIG. 11denotes the default zig-zag scan order typically used for MPEG-2encoding. Listed in this order, the coefficients are C₁₁, C₁₂, C₂₁, C₃₁,C₂₂, C₁₃, C₁₄, C₂₃, C₃₂, C₄₁, . . . C₈₆, C₇₇, C₆₈, C₇₈, C₈₇, C₈₈. Thefirst coefficient in this zig-zag scan order is the DC coefficient C₁₁providing the lowest spatial frequency content in the 8×8 block ofpixels, and the last coefficient in this zig-zag scan order is thecoefficient C₈₈ providing the highest spatial frequency content in the8×8 block of pixels.

FIG. 12 is a diagram illustrating a relationship between an originalMPEG-2 coded bit stream 200 and a reduced-quality MPEG-2 coded bitstream 210 resulting from truncation of high-order DCT coefficients fromthe original MPEG-2 coded bit stream. Shown in the original MPEG-2 codedbit stream 200 is a portion of a video PES packet including DCTcoefficients for an 8×8 pixel block. The DCT coefficients include adifferentially coded DC coefficient 201, and three (run, level) events202, 203, 204 encoding three respective nonzero AC coefficients possiblyalong with some zero valued AC coefficients preceding the three nonzerovalued ones. The DCT coefficients are ordered according to the zig-zagscan order shown in FIG. 11 (or possibly according to an alternatezig-zag scan pattern also supported by the MPEG-2 standard), and ACcoefficients having zero magnitude are described in terms of totalcounts of consecutive zero valued coefficients lying in between twononzero valued coefficients, in the MPEG-2 coded bit stream. Anend-of-block (EOB) code 205 signals the end of the encoded DCTcoefficients for the current block. The reduced-quality MPEG-2 coded bitstream 210 includes a DC coefficient 201′ identical to the DCcoefficient 201 in the original MPEG-2 coded bit stream 200, and a (run,level) event 202′ identical to the (run, level) event 202 in theoriginal MPEG-2 coded bit stream 200. Second and third (run, level)events, however, have been omitted from the reduced-quality MPEG-2 bitstream 210, because an EOB code 205′ immediately follows the (run,level) event 202′. Therefore, the two nonzero high-order AC DCTcoefficients encoded by the second and third (run, level) events 203,204 have been omitted from the reduced-quality MPEG-2 bit stream 210.

FIG. 13 is a flowchart of a procedure for scaling MPEG-2 coded videousing a variety of techniques including the omission of AC DCTcoefficients. The procedure operates upon an original-quality MPEG-2coded video stream by removing AC DCT coefficients in this stream toproduce a lower quality MPEG coded video stream. In a first step 221,execution branches to step 222 if the scaled MPEG coded video is to bespatially subsampled. In step 222, the procedure removes any and all DCTcoefficients for spatial frequencies in excess of the Nyquist frequencyfor the downsampled video. For example, if the low-quality video streamwill be downsampled by a factor of two in both the vertical and thehorizontal directions, then the procedure removes any and all DCTcoefficients having a row index (i) greater than four and any and allDCT coefficients having a column index (j) greater than four. Thisrequires the decoding of the (run, level) coded coefficients to theextent necessary to obtain an indication of the coefficient indices. Ifa sufficient number of the original AC DCT coefficients are removed fora desired bandwidth reduction, then the scaling procedure is finished.Otherwise, execution branches from step 223 to step 224. Execution alsocontinues from step 221 to step 224 if spatial downsampling is notintended.

In step 224, execution branches to step 225 if low-pass scaling isdesired. Low-pass scaling requires the least computational resources andmay produce the best results if the scaled, low-quality MPEG coded videois spatially downsampled. In step 225, the procedure retains up to acertain number of lowest-order AC DCT coefficients for each block andremoves any additional DCT coefficients for each block. This is a kindof frequency domain signal-to-noise ratio scaling (FDSNR) that will bedesignated FDSNR_LP. A specific example of the procedure for step 225will be described below with reference to FIG. 14.

Execution continues from step 224 to step 226 if low-pass scaling is notdesired. In step 226, execution branches to step 227 if largestmagnitude based scaling is desired. Largest magnitude based scalingproduces the least squared error or difference between theoriginal-quality MPEG-2 coded video and the reduced-quality MPEG codedvideo for a given number of nonzero AC coefficients to preserve, but itrequires more computational resources than the low-pass scaling of step225. More computational resources are needed because if there are morenonzero AC coefficients than the desired number of AC coefficients for ablock, then the (run, level) events must be decoded fully to obtain thecoefficient magnitudes, and additional resources are required to findthe largest magnitude coefficients. In step 227, the procedure retainsup to a certain number of largest magnitude AC DCT coefficients for eachblock, and removes any and all additional AC DCT coefficients for eachblock. This is a kind of frequency domain signal-to-noise ratio scaling(FDSNR) that will be designated FDSNR_LM. A specific example of theprocedure for step 227 will be described below with reference to FIG.15.

If in step 226 largest magnitude based scaling is not desired, thenexecution continues to step 228. In step 228, execution branches to step229 to retain up to a certain number of AC DCT coefficients that differin magnitude from up to that number of largest magnitude AC DCTcoefficients by no more than a certain limit. This permits a kind ofapproximation to FDSNR_LM in which an approximate search is undertakenfor the largest magnitude AC DCT coefficients if there are more nonzeroAC DCT coefficients than the desired number of AC DCT coefficients in ablock. The approximate search can be undertaken using a coefficientmagnitude classification technique such as a hashing technique, and thelow-pass scaling technique can be applied to the classification levelthat is incapable of discriminating between the desired number oflargest magnitude AC DCT coefficients. A specific example is describedbelow with reference to FIG. 19.

With reference to FIG. 14, there is shown a flowchart of a procedure forscaling MPEG-2 coded video using the low-pass frequency-domainsignal-to-noise (FDSNR_LP) scaling technique. This procedure scans andselectively copies components of an input stream of original-qualityMPEG-2 coded data to produce an output stream of reduced-quality MPEG-2coded video. The procedure is successively called, and each callprocesses coefficient data in the input stream for one 8×8 block ofpixels. No more than a selected number “k” of coded lowest order(nonzero or zero valued) AC coefficients are copied for the block wherethe parameter “k” can be specified for each block.

In a first step 241 of FIG. 14, the procedure parses and copies thestream of original-quality MPEG-2 coded data up to and including thedifferential DC coefficient variable-length code (VLC). Next, in step242, a counter variable “l” is set to zero. In step 243, the procedureparses the next (run, level) event VLC in the stream of original-qualityMPEG-2 coded data. In step 244, if the VLC just parsed is anend-of-block (EOB) marker, execution branches to step 245 to copy theVLC to the stream of reduced-quality MPEG-2 coded video, and theprocedure is finished for the current block.

In step 244, if the VLC just parsed is not an EOB marker, then executioncontinues to step 246. In step 246, a variable “r” is set equal to therun length of zeroes for the current (run, level) event, in order tocompute a new counter value l+r+1. In step 247, if the new counter valuel+r+1 is greater than the parameter “k”, then the procedure branches tostep 248 to copy an EOB marker to the stream of reduced-quality MPEGcoded data. After step 248, execution continues to step 249, where theprocedure parses the input stream of original-quality MPEG-2 coded datauntil the end of the next EOB marker, and the procedure is finished forthe current block.

In step 247, if the new counter value l+r+1 is not greater than theparameter “k”, then execution continues to step 250. In step 250,execution branches to step 251 if the new counter value l+r+1 is notequal to “k” (which would be the case if the new counter value is lessthan “k”). In step 251, the counter state l is set equal to the newcounter value l+r+1. Then, in step 252, the VLC just parsed (which willbe a VLC encoding a (run,level) event) is copied from the stream oforiginal-quality MPEG-2 coded data to the stream of reduced-qualityMPEG-2 coded data. After step 252, execution loops back to step 243 tocontinue the scanning of the stream of original-quality MPEG-2 codeddata.

In step 250, if the new counter value l+r+1 is equal to “k”, thenexecution branches from step 250 to step 253, to copy the VLC justparsed (which will be a VLC encoding a (run, level) event) from thestream of original-quality MPEG-2 coded data to the stream ofreduced-quality MPEG-2 coded data. Next, in step 254, the procedurecopies an EOB marker to the stream of reduced-quality MPEG-2 coded data.After step 254, execution continues to step 249, where the procedureparses the input stream of original-quality MPEG-2 coded data until theend of the next EOB marker, and the procedure is finished for thecurrent block.

FIG. 15 is a flowchart of a procedure for scaling MPEG-2 coded videousing the largest magnitude based frequency-domain signal-to-noise ratio(FDSNR_LM) scaling technique. This routine is successively called, andeach call processes coefficient data in the input stream for one 8×8block of pixels. No more than a specified number “k” of largestmagnitude AC DCT coefficients are copied for the block, and a differentnumber “k” can be specified for each block.

In a first step 261 in FIG. 15, the procedure parses and copies theinput stream of original-quality MPEG-2 coded data to the output streamof lower-quality MPEG-2 data up to and including the differential DCcoefficient variable-length code (VLC). Then in step 262 all (run,level) event VLCs are parsed and decoded until and including the EOBmarker of the current block. The decoding produces coefficientidentifiers and corresponding quantization indices representing thequantized coefficient values. In step 263, the quantization indices aretransformed to quantized coefficient values. In step 264, the(quantized) coefficients are sorted in descending order of theirmagnitudes. In step 265, the first “k” coefficients of the sorted listare preserved and the last 63-k AC DCT coefficients in the sorted listare set to zero. In step 266, (run, level) event formation and entropycoding (VLC encoding) are applied to the new set of coefficient values.Finally, in step 267, the VLCs resulting from step 266 are copied to theoutput stream until and including the EOB marker.

The sorting step 264 of the FDSNR_LM procedure can consume considerablecomputational resources. It is important to notice that not a fullsorting of the quantized AC coefficients with respect to theirmagnitudes but rather a search for a specified number “k” of largestmagnitude AC coefficients is all that is required. This task can beperformed exactly or approximately in different ways so as to avoid thecomplexity associated with a conventional sorting procedure. In general,a relatively large number of the 63 AC DCT coefficients will have aquantized value of zero. Only the non-zero coefficients need be includedin the sorting process. Moreover, if there are “n” non-zero coefficientsand only “k” of them having the largest magnitudes are to be preservedin the output stream, then the sorting process may be terminatedimmediately after only the largest magnitude “k” coefficients have beenfound, or equivalently immediately after only the smallest magnitude“n−k” coefficients have been found. Moreover, the sorting procedureitself can be different depending on a comparison of “k” to “n” in orderto minimize computations.

With reference to FIG. 16, there is shown a flowchart of a procedurethat selects one of a number of techniques for finding a certain number“k” of largest values out of a set of “n” values. In a first step 271,execution branches to step 272 if “k” is less than ½ “n.” In step 272,execution branches to step 273 if “k” is much less than ½ “n.” In step273, the first “k” values are sorted to produce a list of“k” sortedvalues, and then the last “n−k” values are scanned for any value greaterthan the minimum of the sorted “k” values. If a value greater than theminimum of the sorted “k” values is found, then that minimum value isremoved and the value greater than the minimum value is inserted intothe list of “k” sorted values. At the end of this procedure, the list ofsorted “k” values will contain the maximum “k” values out of theoriginal “n” values. A specific example of this procedure is describedbelow with reference to FIG. 17.

In step 272, if “k” is not much less than ½ “n”, then execution branchesto step 274. In step 274, a bubble-sort procedure is used, including “k”bottom-up bubble-sort passes over the “n” values to put “k” maximumvalues on top of a sorting table. An example of such a bubble-sortprocedure is listed below:

/* TABLE(0) to TABLE(n−1) INCLUDES n VALUES */

/* MOVE THE k LARGEST OF THE n VALUES IN TABLE TO THE RANGE

TABLE(0) TO TABLE(k−1) IN THE TABLE */

/* k<=½ n */

FOR i=1 to k

FOR j=1 to n−i

IF (TABLE(n−j)>TABLE(n−j−1)) THEN(

/* SWAP TABLE(n−j) WITH TABLE(n−j−1) */

TEMP←TABLE(n−j)

TABLE(n−j)←TABLE(n−j−1)

TABLE(n−j−1)←TEMP

NEXT j

NEXT I

In step 271, if “k” is not less than ½ “n”, then execution branches tostep 275. In step 275, if “k” is much greater than ½ “n”, then executionbranches to step 276. In step 276, a procedure similar to step 273 isused, except the “n−k” minimum values are maintained in a sorted list,instead of the “k” maximum values. In step 276, the last “n−k” valuesare placed in the sort list and sorted, and then the first “k” valuesare scanned for any value less than the maximum value in the sortedlist. If a value less than the maximum value in the sorted list isfound, then the maximum value in the sorted list is removed, and thevalue less than this maximum value is inserted into the sorted list. Atthe end of this procedure, the values in the sorted list are the “n−k”smallest values, and the “k” values excluded from the sorted list arethe “k” largest values.

In step 275, if “k” is not much greater than ½ “n”, then executionbranches to step 277. In step 277, a bubble-sort procedure is used,including “n−k” top-down bubble-sort passes over the “n” values to put“n−k” minimum values at the bottom of a sorting table. Consequently, thek maximum values will appear in the top “k” entries of the table. Anexample of such a bubble-sort procedure is listed below:

/* TABLE(0) to TABLE(n−1) INCLUDES n VALUES */

/* MOVE THE n−k SMALLEST OF THE n VALUES IN THE TABLE */

/* TO THE RANGE TABLE(k) TO TABLE(n−1) IN THE TABLE */

/* n>k>=½ n */

FOR i=1 to n−k

FOR j=0 to n−i−1

IF (TABLE(j)<TABLE(j+1)) THEN(

/* SWAP TABLE(j) WITH TABLE(j+1) */

TEMP←TABLE(j)

TABLE(j)←TABLE(j+1)

TABLE(j+1)←TEMP

NEXT j

NEXT I

Turning now to FIG. 17, there is shown a flowchart of a procedure forfinding up to a specified number “k” of largest magnitude AC DCTcoefficients from a set of “n” coefficients, corresponding to theprocedure of FIG. 16 for the case of k<<½ n. In a first step 281, acounter “i” is set to zero. In step 282, the next AC DCT coefficient isobtained from the input stream of original-quality MPEG-2 coded data. Ifan EOB marker is reached, as tested in step 283, then execution returns.In step 284, the counter “i” is compared to the specified number “k”,and if “i” is less than “k”, execution continues to step 285. In step285, a coefficient index and magnitude for the AC DCT coefficient isplaced on a sort list. In step 286, the counter “i” is incremented, andexecution loops back to step 282.

Once the sort list has been loaded with indices and magnitudes for “k”AC DCT coefficients and one additional coefficient has been obtainedfrom the input stream, execution branches from step 284 to step 287. Instep 287 the list is sorted by magnitude, so that the minimum magnitudeappears at the end of the list. Then in step 288 the coefficientmagnitude of the current coefficient last obtained from the input streamis compared to the magnitude at the end of the list. If the coefficientmagnitude of the current coefficient is not greater than the magnitudeappearing at the end of the list, then execution continues to step 289to get the next AC DCT coefficient from the input stream. If an EOBmarker is reached, as tested in step 290, then execution returns.Otherwise, execution loops back to step 288.

In step 288, if the magnitude of the current coefficient is greater thanthe magnitude at the end of the list, then execution branches to step291. In step 291, the entry at the end of the list is removed. In step292, a binary search is performed to determine the rank position of themagnitude of the current coefficient, and in step 293, the currentcoefficient index and magnitude are inserted into the list at the rankposition. The list, for example, is a linked list in the conventionalfashion to facilitate the insertion of an entry for the currentcoefficient at any position in the list. After step 293, execution loopsback to step 288.

An approximation technique of coefficient magnitude classification canbe used to reduce the computational burden of sorting by coefficientmagnitude. A specific example is the use of hashing of the coefficientmagnitude and maintaining lists of the indices of coefficients havingthe same magnitude classifications. As shown in FIG. 18, a hash table300 is linked to hash lists 301 storing the indices of classifiedcoefficients. As shown, the hash table 300 is a list of 2^(M) entries,where “M” is three, and an entry has a value of zero if its associatedlist is empty, and otherwise the entry has a pointer to the end of thecoefficients in its associated list. The lists shown in FIG. 18 havefixed memory allocations in which the pointers in the hash table alsoindicate the number of coefficient indices in the respective hash lists.Alternatively, the hash lists could be dynamically allocated and linkedin the conventional fashion.

FIG. 19 shows a flowchart of a procedure for using the hash table 300and hash lists 301 of FIG. 18 to perform a sort of “k” coefficientshaving approximately the largest magnitudes from a set of “n”coefficients. This approximation technique ensures that none of the “k”coefficients selected will have a magnitude that differs by more than acertain error limit from the smallest magnitude value of “k”coefficients having the largest magnitude. The error limit isestablished by the number of hash table entries, and it is the range ofthe magnitudes that can be hashed to the same hash table entry.

In a first step 311 in FIG. 19, the hash table is cleared. Then in step312, the next AC DCT coefficient is obtained from the input stream. Ifan EOB marker is not reached, as tested in step 313, then executioncontinues to step 314. In step 314, a hash table index is stripped fromthe most significant bits (MSBs) of the coefficient magnitude. For thehash table in FIG. 18 having eight entries, the three most significantbits of the coefficient magnitude are stripped from the coefficientmagnitude. This is done by a bit masking operation together with alogical arithmetic shift operation. Then in step 315, the coefficientindex is inserted on the hash list of the indexed hash table entry. Forexample, the hash table entry is indexed to find the pointer to wherethe coefficient index should be inserted, and then the pointer in thehash table entry is incremented. After step 315, execution loops back tostep 312. Once all of the AC coefficients for the block have beenclassified by inserting them in the appropriate hash lists, an EOBmarker will be reached, and execution will branch from step 313 to step316.

Beginning in step 316, the hash table and hash lists are scanned to findapproximately the “k” largest magnitude coefficients. The hash listslinked to the bottom entries of the hash table will have the indices jor the largest magnitude coefficients. Each hash list is scanned fromits first entry to its last entry, so that each hash list is accessed asa first-in-first-out queue. Therefore, in each magnitude classification,the coefficient ordering in the output stream will be the same as thecoefficient ordering in the input stream, and the approximation willhave a “low pass” effect in which possibly some lower-frequencycoefficients having slightly smaller magnitudes will be retained at theexpense of discarding some higher-frequency coefficients having slightlylarger magnitudes. (The approximation results from the fact that thelast hash list to be scanned is not itself sorted, and to eliminate theerror of the approximation, the last hash list to be scanned could besorted.)

In step 316, a scan index “i” is set to 2^(M)−1 in order to index thehash table beginning at the bottom of the table, and a counter “j” isset equal to “k” in order to stop the scanning process after finding “k”coefficients. Next, in step 317, the hash table is indexed with “i”. Instep 318, if the indexed entry of the hash table is zero, then executionbranches to step 319. In step 319, the procedure is finished if “i” isequal to zero; otherwise, execution continues to step 320. In step 320,the index “i” is decremented, and execution loops back to step 317.

If in step 318 the indexed hash table entry is not zero, then executioncontinues to step 321. In step 321, the next entry is obtained from theindexed hash list, and the coefficient index in the entry is used to putthe indexed coefficient in the output stream. Then in step 322 executionbranches to step 319 if the end of the indexed hash list is reached inthe previous step 321. If the end of the list was not reached in step321, then execution continues from step 322 to step 323. In step 323 thecounter “j” is decremented, and in step 324 the counter “j” is comparedto zero. In step 324, if the counter “j” is less than or equal to zero,then the procedure is finished. Otherwise, if the counter “j” is notless than or equal to zero in step 324, execution loops back to step321.

The FDSNR_LM procedure, as described above, in general provides asignificant improvement in peak signal-to-noise ratio (PSNR) over theFDSNR_LP procedure when each procedure retains the same number ofnon-zero AC DCT coefficients. It has been found, however, thatsubstantially more bits are required for the (run, level) coding of thenon-zero AC DCT coefficients resulting from the FDSNR_LM procedure thanthose resulting from the FDSNR_LP procedure, provided that the samecoefficient quantization and scanning method is used. Therefore, theFDSNR_LM procedure provides at best a marginal improvement inrate-distortion (PSNR as a function of bit rate) over the FDSNR_LPprocedure unless the non-zero AC DCT coefficients for the FDSNR_LMprocedure are quantized, scanned, and/or (run, level) coded in a fashiondifferent from the quantization, scanning, and/or (run, level) coding ofthe coefficients in the original MPEG-2 clip. A study of this problemresulted in a discovery that it is sometimes possible to reduce thenumber of bits for (run, level) coding of coefficients for an 8×8 blockincluding a given number of the non-zero largest magnitude AC DCTcoefficients if additional coefficients are also (run, level) coded forthe block.

The (run, level) coding of the non-zero AC DCT coefficients from theFDSNR_LM procedure has been found to require more bits than from theFDSNR_LP procedure due to an increased occurrence frequency of escapesequences for the (run, level) coding. The increased frequency of escapesequences is an indication that the statistical likelihood of possible(run, level) combinations for the non-zero AC DCT coefficients selectedby the FDSNR_LM procedure is different from the statistical likelihoodof possible (run, level) combinations for the non-zero AC DCTcoefficients produced by the standard MPEG-2 coding process and inparticular those selected by the FDSNR_LP procedure.

The MPEG-2 coding scheme assigns special symbols to the (run, level)combinations that occur very frequently in ordinary MPEG-2 coded video.The most frequent (run, level) combinations occur for short run lengths(within the range of about 0 to 5, where the run length can range from 0to 63) and relatively low levels (about 1 to 10, where the level canrange from 1 to 2048). The most frequent of these special symbols areassigned the shortest variable-length code words (VLCs). If a (run,level) combination does not have such a special symbol, then it is codedas an escape sequence including a 6-bit escape sequence header code wordfollowed by a 6-bit run length followed by a 12 bit signed level. Anescape sequence requires a much greater number of bits than the specialsymbols, which have varying lengths depending on their relativefrequency. In particular, the escape sequences each have 24 bits, andthe special symbols have variable-length code words having a maximum of17 bits.

There are two (run, level) VLC tables. The first coding table isdesignated TABLE 0, and the second is designated TABLE 1. These tablesspecify the (run, level) combinations having special symbols, and thespecial symbol for each such combination. For each table, the (run,level) combinations having special symbols, and the range of the VLC bitlengths of the special symbols, are summarized below:

SUMMARY OF PROPERTIES OF DCT COEFFICIENT TABLE ZERO (Table Zero is TableB. 14, p. 135 of ISO/IEC 13818-2 1996E)

Run Range of Levels Range of Code Lengths 0  1 to 40 2 to 16 1  1 to 184 to 17 2 1 to 5 5 to 14 3 1 to 4 6 to 14 4 1 to 3 6 to 13 5 1 to 3 7 to14 6 1 to 3 7 to 17 7 1 to 2 7 to 13 8 1 to 2 8 to 13 9 1 to 2 8 to 1410 1 to 2 9 to 14 11 1 to 2 9 to 17 12 1 to 2 9 to 17 13 1 to 2 9 to 1714 1 to 2 11 to 17  15 1 to 2 11 to 17  16 1 to 2 11 to 17  17 1 13 18 113 19 1 13 20 1 13 21 1 13 22 1 14 23 1 14 24 1 14 25 1 14 26 1 14 27 117 28 1 17 29 1 17 30 1 17 31 1 17

SUMMARY OF PROPERTIES OF DCT COEFFICIENT TABLE ONE (Table One is TableB. 15, p. 139 of ISO/IEC 13818-2 1996E)

Run Range of Levels Range of Code Lengths 0  1 to 40 3 to 16 1  1 to 184 to 17 2 1 to 5 6 to 14 3 1 to 4 6 to 14 4 1 to 3 7 to 13 5 1 to 3 7 to14 6 1 to 3 8 to 17 7 1 to 2 8 to 13 8 1 to 2 8 to 13 9 1 to 2 8 to 1410 1 to 2 8 to 14 11 1 to 2 9 to 17 12 1 to 2 9 to 17 13 1 to 2 9 to 1714 1 to 2 10 to 17  15 1 to 2 10 to 17  16 1 to 2 11 to 17  17 1 13 18 113 19 1 13 20 1 13 21 1 13 22 1 14 23 1 14 24 1 14 25 1 14 26 1 14 27 117 28 1 17 29 1 17 30 1 17 31 1 17

The FDSNR_LP procedure selected AC DCT coefficients have (run, level)symbol statistics that are similar to the statistics of ordinary MPEG-2coded video, and therefore the FDSNR_LP AC DCT coefficients have asimilar frequency of occurrence for escape sequences in comparison tothe ordinary MPEG-2 coded video. In contrast, the FDSNR_LM procedureselects AC DCT coefficients resulting in (run, level) combinations thatare less likely than the combinations for ordinary MPEG-2 coded video.This is due to two reasons. First, the FDSNR_LM procedure selects AC DCTcoefficients having the highest levels. Second, the FDSNR_LM procedureintroduces higher run lengths due to the elimination of coefficientsover the entire range of coefficient indices. The result is asignificantly increased rate of occurrence for escape sequences. Escapesequences form the most inefficient mode of coefficient informationencoding in MPEG-2 incorporated into the standard so as to coverimportant but very rarely occurring coefficient information.

In order to improve the rate-distortion performance of thescaled-quality MPEG-2 coded video from the FDSNR_LM procedure, thenon-zero AC DCT coefficients selected by the FDSNR_LM procedure shouldbe quantized, scanned, and/or (run, level) coded in such a way thattends to reduce the frequency of the escape sequences. For example, ifthe original-quality MPEG-2 coded video was (run, level) coded usingTABLE 0, then the largest magnitude coefficients should be re-codedusing TABLE 1 because TABLE 1 provides shorter length VLCs for some(run, level) combinations having higher run lengths and higher levels.It is also possible that re-coding using the alternate scan methodinstead of the zig-zag scan method may result in a lower frequency ofoccurrence for escape sequences. For example, each picture could be(run, level) coded for both zig-zag scanning and alternate scanning, andthe scanning method providing the fewest escape sequences, or the leastnumber of bits total, could be selected for the coding of thereduced-quality coded MPEG video.

There are two methods having general applicability for reducing thefrequency of escape sequences resulting from the FDSNR_LM procedure. Thefirst method is to introduce a non-zero, “non-qualifying” AC DCTcoefficient of the 8×8 block into the list of non-zero qualifying AC DCTcoefficients to be coded for the block. In this context, a “qualifying”coefficient is one of the k largest magnitude coefficients selected bythe FDSNR_LM procedure. The non-qualifying coefficient referred toabove, must be lying in between two qualifying AC DCT coefficients (inthe coefficient scanning order) that generate the (run, level)combination causing the escape sequence. Moreover, this non-qualifyingcoefficient must cause the escape sequence to be replaced with twoshorter length VLCs when the AC DCT coefficients are (run, level) coded.This first method has the effect of not only decreasing the number ofbits in the coded reduced-quality MPEG video in most cases, but alsoincreasing the PSNR.

The qualifying AC DCT coefficient causing the escape sequence that isfirst in the coefficient scanning order will be simply referred to asthe first qualifying coefficient. The qualifying AC DCT coefficientcausing the escape sequence that is second in the coefficient scanningorder will be simply referred to as the second qualifying coefficient.For example, suppose the qualifying coefficients in zig-zag scan orderfor an 8×8 block include C₅₁ followed by C₁₅ having a level of 40. Ifonly the qualifying coefficients were (run, level) coded for themicroblock, C₁₅ would result in a run length of 3, because there are atotal of three non-qualifying coefficients (C₄₂, C₃₃, and C₂₄) betweenC₅₁ and C₁₅ in the scan order. Therefore, C₁₅ would have to be coded asan escape sequence, because a run of 3 and level of 40 does not have aspecial symbol. In this example, the escape sequence is in effect causedby a first qualifying coefficient, which is C₅₁, and a second qualifyingcoefficient, which is C₁₅. This escape sequence can possibly beeliminated say, if C₂₄ is a non-zero, non-qualifying coefficient of theblock, C₂₄ has a level of 5 or less, and C₂₄ is (run, level) codedtogether with the qualifying coefficients. For example, assuming thatC₂₄ has a level of 5, and using the MPEG-2 (run, level) coding TABLE 1,then C₂₄ has a run length of two and is coded as the special symbol 00000000 1010 0s, where “s” is a sign bit, and C₁₅ now has a run length of 0and is coded as the special symbol 0000 0000 0010 00s. Such aconsideration clearly applies to the rest of the non-zero non-qualifyingcoefficients lying in between the two qualifying coefficients producingthe escape sequence. In the above example, these non-qualifyingcoefficients are C₄₂ and C₃₃.

Whether or not an escape sequence can be eliminated from the (run,level) coding of the qualifying coefficients can be determined bytesting a sequence of conditions. The first condition is that the secondqualifying coefficient must have a level that is not greater than themaximum level of 40 for the special (run, level) symbols. If thiscondition is satisfied, then there must be a non-zero non-qualifying ACDCT coefficient that is between the first and second qualifyingcoefficients in the coefficient scanning order. If there is such anon-qualifying coefficient, then the combination of its level and therun length between the first qualifying coefficient and thenon-qualifying coefficient in the coefficient scanning order must be oneof the special (run, level) symbols. If so, then the combination of thelevel of the second qualifying coefficient and the run length betweenthe non-qualifying coefficient and the second qualifying coefficientmust also be a special (run, level) symbol, and if so, all requiredconditions have been satisfied. If not, then the conditions with respectto the non-qualifying coefficient are successively applied to any othernon-zero non-qualifying AC DCT coefficient of the block lying in betweenthe two qualifying coefficients, until either all conditions are foundto be satisfied or all such non-qualifying coefficients are tested andfailed. If there are sufficient computational resources, this searchprocedure should be continued to find all such non-qualifyingcoefficients that would eliminate the escape sequence, and to select thenon-qualifying coefficient that converts the escape sequence to the pairof special symbols having respective code words that in combination havethe shortest length.

A flow chart for a modified FDSNR_LM procedure using the first method isshown in FIGS. 20 and 21. In a first step 331 of FIG. 20, the procedurefinds up to “k” largest magnitude non-zero AC DCT coefficients (i.e.,the “qualifying coefficients”) for the block. (This first step 331 issimilar to steps 261 to 265 of FIG. 15, as described above.) In step332, (run, level) coding of the qualifying coefficients is begun in thescan order using the second coding table (Table 1). This (run, level)coding continues until an escape sequence is reached in step 333, or theend of the block is reached in step 336. If an escape sequence isreached, execution branches from step 333 to step 334. If the level ofthe second qualifying coefficient causing the escape sequence is greaterthan 40, execution continues from step 334 to step 336. Otherwise,execution branches from step 334 to step 335 to invoke a subroutine (asfurther described below with reference to FIG. 21) to possibly include anon-zero non-qualifying AC DCT coefficient in the (run, level) coding toeliminate the escape sequence. The subroutine either returns withoutsuccess, or returns such a non-qualifying coefficient so that the escapesequence is replaced with the two new (run, level) codings of the firstqualifying coefficient and the non-qualifying coefficient and then thenon-qualifying coefficient and the second qualifying coefficient. Fromstep 335, execution continues to step 336. Execution returns from step336 if the end of the block is reached. Otherwise, execution continuesfrom step 336 to step 337, to continue (run, level) coding of thequalifying coefficients in the scan order using the second coding table(TABLE 1). This (run, level) coding continues until an escape sequenceresults, as tested in step 333, or until the end of the block isreached, as tested in step 336.

With reference to FIG. 21, there is shown a flow chart of the subroutine(that was called in step 335 of FIG. 20) for attempting to find anon-zero, non-qualifying AC DCT coefficient that can be (run, level)coded to eliminate an escape sequence for a qualifying coefficient. In afirst step 341, the procedure identifies the first qualifyingcoefficient and the second qualifying coefficient causing the escapesequence. For example, the subroutine of FIG. 21 can be programmed as afunction having, as parameters, a pointer to a list of the non-zero ACDCT coefficients in the scan order, an index to the first qualifyingcoefficient in the list, and an index to the second qualifyingcoefficient in the list. In step 342, the subroutine looks for anon-zero non-qualifying AC DCT coefficient between the first and thesecond qualifying coefficients in the scan order. For example, the valueof the index to the first qualifying coefficient is incremented andcompared to the value of the index for the second qualifyingcoefficient, and if they are the same, there is no such non-qualifyingcoefficient. Otherwise, if the new coefficient pointed to (byincrementing the index of the first qualifying coefficient) is anon-zero coefficient then it becomes a candidate non-qualifyingcoefficient deserving further testing. If however the new coefficientpointed to (by incrementing the index of the first qualifyingcoefficient) has a value zero then it is not a candidate non-qualifyingcoefficient. If no such (candidate) non-qualifying coefficients arefound, as tested in step 343, then execution returns from the subroutinewith a return code indicating that the search has been unsuccessful.Otherwise, execution continues to step 344.

In step 344, the non-qualifying coefficient is (run, level) coded, todetermine in step 345 whether it codes to an escape sequence. If itcodes to an escape sequence, then execution loops back from step 345 tostep 342 to look for another non-zero non-qualifying AC DCT coefficientin the scan order between the first and second qualifying coefficients.If it does not code to an escape sequence, then execution continues fromstep 345 to step 346. In step 346, the second qualifying coefficient is(run, level) coded, using the new run length, which is the number ofcoefficients in the scan order between the non-qualifying coefficientand the second qualifying coefficient. If it codes to an escapesequence, as tested in step 347, then execution loops back from step 347to step 342 to look for another non-zero non-qualifying AC DCTcoefficient in the scan order between the first and second qualifyingcoefficients. If it does not code to an escape sequence, then executioncontinues from step 347 to step 348.

In step 348, execution returns with a successful search result unless acontinue search option has been selected. If the continue search optionhas been selected, then execution branches from step 348 to step 349 tosearch for additional non-zero non-qualifying AC DCT coefficients thatwould eliminate the escape sequence. In other words, steps 342 to 347are repeated in an attempt to find additional non-zero non-qualifying ACDCT coefficients that would eliminate the escape sequence. If no moresuch non-qualifying coefficients are found, as tested in step 350,execution returns with a successful search result. Otherwise, executionbranches from step 350 to step 351 to select the non-qualifyingcoefficient giving the shortest overall code word length and/or thelargest magnitude for the best PSNR, and execution returns with asuccessful search result. For example, for each non-qualifyingcoefficient that would eliminate the escape sequence, the total bitlength is computed for the (run, level) coding of the non-qualifyingcoefficient and the second qualifying coefficient. Then a search is madefor the non-qualifying coefficient producing the shortest total bitlength, and if two non-qualifying coefficients which produce the sametotal bit length are found, then the one having the largest level isselected for the elimination of the escape sequence.

A second method of reducing the frequency of occurrence of the escapesequences in the (run, level) coding of largest magnitude AC DCTcoefficients for an 8×8 block is to change the mapping of coefficientmagnitudes to the levels so as to reduce the levels. Reduction of thelevels increases the likelihood that the (run, level) combinations willhave special symbols and therefore will not generate escape sequences.This second method has the potential of achieving a greater reduction inbit rate than the first method, because each escape sequence can now bereplaced by the codeword for one special symbol, rather than by the twocodewords as is the case for the first method. The second method,however, may reduce the PSNR due to increased quantization noiseresulting from the process producing the lower levels. Therefore, if adesired reduction of escape sequences can be achieved using the firstmethod, then there is no need to perform the second method, which islikely to reduce the PSNR. If the first method is used but not all ofthe escape sequences have been eliminated, then the second method couldbe used to possibly eliminate the remaining escape sequences.

The mapping of coefficient magnitudes to the levels can be changed bydecoding the levels to coefficient magnitudes, changing the quantizationscale factor (qsi), and then re-coding the levels in accordance with thenew quantization scale factor (qsi). The quantization scale factor isinitialized in each slice header and can also be updated in themacroblock header on a macroblock basis. Therefore it is a constant forall blocks in the same macroblock In particular, the quantization scalefactor is a function of a q_scale_type parameter and aquantizer_scale_code parameter. If q_scale_type=0, then the quantizerscale factor (qsi) is twice the value of q_scale_code. Ifq_scale_type=1, then the quantizer scale factor (qsi) is given by thefollowing table, which is the right half of Table 7-6 on page 70 ofISO/IEC 13838-2:1996(E):

quantizer scale code quantization scale factor (qsi) 1 1 2 2 3 3 4 4 5 56 6 7 7 8 8 9 10 10 12 11 14 12 16 13 18 14 20 15 22 16 24 17 28 18 3219 36 20 40 21 44 22 48 23 52 24 56 25 64 26 72 27 80 28 88 29 96 30 10431 112

In a preferred implementation, to reduce the coefficient levels, thequantization scale factor is increased by a factor of two, and thelevels of the non-zero AC DCT coefficients are reduced by a factor oftwo, so long as the original value of the quantization scale factor isless than or equal to one-half of the maximum possible quantizationscale factor. For q_scale_type=1, a factor of two increase in thequantization scale factor (qsi) is most easily performed by a tablelookup of a new quantization_scale_code using the following conversiontable:

Original quantization scale code New quaitization scale code 1 2 2 4 3 64 8 5 9 6 10 7 11 8 12 9 14 10 16 11 17 12 18 13 19 14 20 15 21 16 22 1724 18 25 19 26 20 27 21 28 22 29 23 30 24 31

In a preferred method for generation of trick mode files, thequantization scale factor is adjusted in order to achieve a desiredreduction in the escape sequence occurrence frequency resulting from themodified FDSNR_LM procedure, and the number (k) of largest magnitudecoefficients is adjusted in order to achieve a desired reduction in bitrate. A specific implementation is shown in the flow chart of FIGS.22-23. In a first step 361, the number (k) of largest magnitude ACcoefficients per 8×8 block is initially set to a value of 9, and thequantization scaling factor (QSF) is initially set to a value of 2. Thenconversion of the I frames of an original-quality MPEG-2 coded videoclip to a lower quality level begins. When a picture header isencountered in step 362, indicating the beginning of a new I frame,execution continues to step 363. In step 363, execution branchesdepending on the value of the intra_vlc_format parameter in the pictureheader of the original-quality MPEG-2 coded video clip. This value iseither 0, indicating that the first (run, level) coding table (TABLE 0)was used for coding the picture, or 1, indicating that the second (run,level) coding table (TABLE 1) was used for coding the picture. In eithercase, the down scaled quality picture will be coded with the second(run, level) coding table. If the intra_vlc_format parameter is equal to0 execution continues from step 363 to step 364 where TABLE 0 is read infor (run, level) symbol decoding in the original-quality MPEG-2 codedclip. Otherwise, if the intra_vlc_format parameter is equal to 1, thenexecution continues from step 363 to step 365 where TABLE 1 is read infor (run, level) symbol decoding in the original-quality MPEG-2 codedclip.

After steps 364 and 365, execution continues to step 366. In step 366,the modified FDSNRS_LM procedure is applied to the 8×8 blocks of thecurrent slice, using the adjusted quantization scale index, if theadjusted quantization scale index is less than the maximum possiblequantization scale index. In step 367, execution loops back to step 362to continue 8×8 block conversion until a new slice header isencountered, indicating the beginning of a new slice. Once a new sliceis encountered, execution continues from step 367 to step 368. In step368, the average escape sequence occurrence frequency per block for thelast slice is compared to a threshold TH1. If the escape sequenceoccurrence frequency is greater than the threshold, then executionbranches to step 369. In step 369, if the quantization scaling factor(QSF) is less than or equal to a limit value such as 2, then executionbranches to step 370 to increase the quantization scaling factor (QSF)by a factor of two.

In step 368, if the escape sequence occurrence frequency is not greaterthan the threshold TH1, then execution continues to step 371 of FIG. 23.In step 371, the average escape sequence occurrence frequency per 8×8block for the last slice is compared to a threshold TH2. If the escapesequence occurrence frequency is less than the threshold TH2, thenexecution branches to step 372. In step 372, if the quantization scalingfactor (QSF) is greater than or equal to a limit value such as 2, thenexecution branches to step 373 to decrease the quantization scalingfactor (QSF) by a factor of two. After step 373, and also after step 370of FIG. 22, execution continues to step 374 of FIG. 23. In step 374,execution continues to step 375 if a backtrack option has been selected.In step 375, re-coding for the last slice is attempted using theadjusted quantization scale factor. The new coding, or the coding thatgives the best results in terms of the desired reduction of escapesequence occurrence frequency, is selected for use in the scaled qualitypicture. After step 375, execution continues to step 376. Execution alsocontinues to step 376 from: step 369 in FIG. 22 if the quantizationscaling factor (QSF) is not less than or equal to 2; step 371 in FIG. 23if the escape sequence occurrence frequency is not less than thethreshold TH2; step 372 in FIG. 23 if the quantization scaling factor(QSF) is not greater than or equal to 2; and from step 374 in FIG. 23 ifthe backtrack option has not been selected.

In step 376, the average bit rate of the (run, level) coding per 8×8block for at least the last slice is compared to a high threshold TH3.Preferably this average bit rate is a running average over the alreadyprocessed segment of the current scaled quality I-frame, and the highthreshold TH3 is selected to prevent video buffer overflow in accordancewith the MPEG-2 Video Buffer Verifier restrictions. If the average bitrate exceeds the high threshold TH3, then execution continues to step377, where the number (k) of non-zero largest magnitude AC coefficientsper 8×8 block is compared to a lower limit value such as 6. If thenumber (k) is greater than or equal to 6, then execution continues tostep 378 to decrement the number (k).

In step 376, if the average bit rate is not greater than the thresholdTH3, then execution continues to step 379. In step 379, the average bitrate is compared to a lower threshold TH4. If the average bit rate isless than the threshold TH4, then execution branches from step 379 tostep 380, where the number (k) of non-zero largest magnitude AC DCTcoefficients per 8×8 block is compared to a limit value of 13. If thenumber (k) is less than or equal to 13, then execution continues to step381 to increment the number (k). After step 378 or 381, executioncontinues to step 382. In step 382, execution continues to step 383 if abacktrack option is selected. In step 383, an attempt is made to re-codethe last slice for the scaled quality picture using the adjusted valueof the number (k) of non-zero largest magnitude AC DCT coefficients perblock. After step 383, execution loops back to step 362 of FIG. 22 tocontinue generation of the scaled quality clip. Execution also loopsback to step 362 of FIG. 22 after: step 377 if the value of (k) is notgreater than or equal to 6; step 379 if the average bit rate is not lessthan the threshold TH4; step 380 if the value of (k) is not less than orequal to 13; and step 382 if the backtrack option has not been selected.Coding of the scaled quality clip continues until the end of theoriginal quality clip is reached in step 364 of FIG. 22, in which caseexecution returns.

In a preferred implementation, a fast forward trick mode file and a fastreverse trick mode file are produced from an original-quality MPEG-2coded video main file when the main file is ingested into the video fileserver. As shown in FIG. 24, a volume generally designated 390 isallocated to store the main file 391. The volume 390 includes anallocated amount of storage that exceeds the real file size of the mainfile 391 in order to provide additional storage for meta-data 392, thefast forward trick file 393, and the fast reverse trick file 394. Thetrick files are not directly accessible to clients as files; instead,the clients may access them thorough trick-mode video service functions.With this strategy, the impact on the asset management is a minimum. Nomodification is needed for delete or rename functions.

Because the volume allocation is done once for the main file and itsfast forward and fast reverse trick mode files, there is no risk of lackof disk space for production of the trick files. The amount of diskblocks to allocate for these files is computed by the video serviceusing a volume parameter (vsparams) specifying the percentage of size toallocate for trick files. A new encoding type is created in addition totypes RAW for direct access and MPEG2 for access to the main file. Thenew encoding type is called EMPEG2, for extended MPEG2, for reference tothe main file plus the trick files. The video service allocates theextra file size only for these files.

For the transfer of these files to archive or to another video fileserver, it would be useful to transfer all the data even if it is anon-standard format. For the FTP copy-in, a new option is added tospecify if the source is in the EMPEG2 format or if it is a standardMPEG2 file. In the first case, the copy-in should provide the completefile 390. In the second case, the video service allocates the extra sizeand the processing is the same as for a record. For the copy-out, thesame option can be used to export the complete file 390 or only the mainpart 391. The archiving is always done on the complete file 390.

The trick mode file production is done by a new video service procedure.This procedure takes as input the speed-up factor (or the target trickmode file size) along with the number of freeze (P or B) frames toinsert in between the scaled I frames and then generates both the fastforward file 393 and the fast reverse file 394 for this speed-up factor(or target trick mode file size) and with the specified number ofinterleaving freeze frames. Since the bandwidth of the original clip (inthe main file) and the bandwidths of the two trick mode clips (in thefast forward and fast reverse files) are the same, the speed-up factorand the target trick mode file size are equivalent pieces ofinformation. A default speed-up factor (system parameter) can be used.The main file is read and the trick mode files are produced. If a trickmode file already exists with the same speed-up factor, it is rewrittenor nothing is done depending on an option. Multiple trick mode filescould be created with different speed-up factors. But it is preferred topermit only one set of fast forward and fast reverse trick mode files tobe produced at a time (i.e., no parallel generation with differentspeed-up factors). The current speed-up factor is a parameter of thevolume parameters (vsparams).

As stated above another parameter to be provided to the video serviceprocedure in charge of trick mode file generation is the number offreeze frames to be inserted in between consequent scaled I frames. Thepreferred values for this parameter are 0 and 1, although other positiveinteger values greater than I are also possible. The inclusion of freezeframes due to their very small sizes spare some bandwidth which can thenbe used to improve the quality of scaled I frames. Hence, the freezeframes in this context provide a mechanism to achieve a trade-offbetween the scaled I frame quality and the temporal (motion) sampling.Depending on the speed-up factor (or the target trick mode file size)and also the number of interleaving freeze frames to be inserted, thevideo service procedure in charge of trick mode file generationdetermines a sub-sampling pattern (closest to uniform) to choose theoriginal I frames which will be scaled and included in the trick modefiles. For example, the case of an original clip with 10 frames per GOP,a trick mode file size which is 10% of the main file together with 0freeze frames, implies the use of all original I frames for being scaledand included in the trick mode file. This will typically result in a lowquality scaling. As another example, the case of an original clip with10 frames per GOP, a trick mode file size which is 10% of the main filetogether with I freeze frame, implies the use of a 2 to 1 (2:1)sub-sampling on the original I frames which will choose every otheroriginal I frame for being scaled and included in the trick mode file.

FIG. 25 is a more detailed diagram of the volume 390, showing additionalmeta-data and related data structures. The I node 401 includes 4 diskblocks containing a file-system oriented description of the file. TheMeta-data (MD) directory 402 includes 4 disk blocks describing eachentry of the meta-data area 392. The entries of the meta-data area 392include a description of the MPEG-2 meta-data 403, a description of thetrick files header meta-data 404, and a description of the GOP indexmeta-data 405. The MPEG-2 meta-data 403 includes 15 disk blocks maximum.

The trick files header 404 includes I disk block, which specifies thebeginning of free area (end of last trick file) in blocks, the number oftrick files couple (FF FR), and for each trick file, a speed-up factor,a block address of the GOP index, a block address of the trick fileforward, a byte length of the trick file forward, a block address of thetrick file reverse, a byte length of the trick file reverse, a framesnumber of the trick file, and a number of GOP of each trick files.

The GOP index includes 2024 disk blocks. The GOP index specifies, foreach GOP, a frame number, a pointer to the MPEG-2 data for the GOP inthe main file, and various flags and other attributes of the GOP. Theflags indicate whether the GOP entry is valid and whether the GOP isopen or closed. The other attributes of the GOP include the maximum bitrate, the average bit rate, the AAU size in bytes, the APU duration inseconds, the audio PES packet starting locations, the AAU startinglocations, the AAU PTS values, and the decode time stamp (DTS) and thevalue of the program clock reference (PCR) extrapolated to the firstframe of the GOP. The size of all the data preceding the main file is,for example, 1 megabyte.

There is one GOP index 406 for both the fast forward file 393 and thefast reverse file 394. The GOP index 406 of the trick files is differentthan the GOP index 405 of the main file. The GOP index 406 of the trickfiles contains, for each GOP, the byte offset in the trick file forwardof the TS packet containing the first byte of the SEQ header, the framenumber in the fast forward file of the GOP (the same value for the fastreverse file can be computed from this value for the fast forward file),the frame number in the original file of the first frame of the GOP, andthe byte offset in the original file of the same frame (to resume afterfast forward or reverse without reading the main GOP index).

The GOP index 405 for the main file and the GOP index 406 for the fastforward and fast reverse trick files provides a means for rapidlyswitching between the normal video-on-demand play operation during thereading of the main file, and the fast-forward play during the readingof the fast-forward file, and the fast-reverse play during the readingof the fast reverse file. For example, FIG. 26A illustrates the readaccess to various GOPs in the main file, fast forward file, and fastreverse file, during a play sequence listed in FIG. 26B. Due to thepresence of down-scaled I frames and possibly consequent freeze framesin the trick mode files, the video buffer verifier (VBV) model for atrick mode file is different than the VBV model of the main file.Consequently, the mean video decoder main buffer fullness levels can besignificantly different for these files. For example, a transition fromthe main file to one of the trick files will usually involve adiscontinuity in the mean video decoder main buffer fullness level,because only the I frames of the main file correspond to frames in thetrick files, and the corresponding I frames have different bit rateswhen the trick mode I frames are scaled down for a reduced bit rate. Aninstantaneous transition from a trick file back to the main file mayalso involve a discontinuity especially when freeze frames are insertedbetween the I frames for trick mode operation. To avoid thesediscontinuities, the seamless splicing procedure of FIGS. 3 to 6 asdescribed above is used during the transitions from regular play modeinto trick mode and similarly from trick mode back into the regular playmode. Through the use of the seamless splicing procedure to modify thevideo stream content, for example for the “Seamless Splice” locationsidentified in FIG. 26A, the video decoder main buffer level will bemanaged so as to avoid both overflows and underflows leading to visualartifacts.

It is desired to copy in and out of the volume 390 with or without themeta-data 392 and the trick files 393, 394. This is useful to exportand/or import complete files without regenerating the trick files. Thefile encoding type is now recognized as a part of the volume name.Therefore there can be multiple kinds of access to these files. The readand write operations are done by derivations of the class file systeminput/output (FSIO) which takes into account the proper block offset ofthe data to read or write. There is one derivation of FSIO per encodingtype, providing three different access modes. EMGP3, MPEG2, and RAW.EMPEG2 accesses the whole volume from the beginning of the meta-dataarray, and in fact provides access to the entire volume except the inode 401, but no processing is done. MPEG2 access only the main part ofthe asset with MPEG processing, including file analyze and meta-datageneration in a write access. RAW access only the main part of the assetwithout processing. These access modes are operative for read and writeoperations for various access functions as further shown in FIG. 27.

During a record operation, the video service allocates a volume andcomputes the number of block to allocate using the volume parametergiving the percentage to add for the trick files. Then, the size inblocks given to the stream server is the main part size only without theextension for the trick files. This avoids using the reserved part ofthe volume when the effective bit rate is higher than the requested bitrate. At the end of a record operation or a FTP copyin operation, thevideo service calls a procedure CMSPROC_GETATTR, and the stream serverreturns the actual number of bytes received and the actual number ofblocks used by the main file plus the meta-data. The same values arereturned for both MPEG2 and EMPEG2 files. The video service computesagain the file extension to manage the trick files and adjust the numberof allocated blocks.

Both trick files forward and reverse are generated by the same command.First, the trick file forward is generated by reading the main file. Thetrick file GOP index is built and kept in memory. During thisgeneration, only the video packets are kept. PCR, PAT and PMT will beregenerated by the MUX in play as for any other streams. The audiopackets are discarded. This ensures that there is enough stuffingpackets for the PCR reinsertion. For this, a stuffing packet is insertedevery 30 milliseconds.

Then using the GOP index, the trick file forward is read GOP by GOP inreverse order to generate the trick file reverse. The same GOPs arepresent in both files. The only modification done is an update of thevideo PTS, which must be continuous. Then, the GOP index is written ondisk. This avoids reading again the file while generating the secondtrick file. The GOP index size is: 24 times the GOP number. In the worstcase (the file is assumed not to be I frame only), there are 2 framesper GOP and 30 frames per second. So for 1 hour in fast forward, the GOPindex size is: (24×3600×30)/2=1296000 bytes. This will be the case for a4 hour film played at 4 times the normal speed. Therefore, this GOPindex can be kept in memory during the trick file generations withoutrisk of memory overflow.

The read and write rate are controlled to conserve bandwidth on thecached disk array. The bandwidth reserved for these generations is aparameter given by the video service. It is a global bandwidth for bothread and writes. The number of disk 1/0 per seconds is counted so as notto exceed this bandwidth.

The trick files header update is done once when both the fast forwardand fast reverse trick files and the GOP index have been successfullywritten.

Playing a file is done with the CM_MpegPlayStream class. Fast forward(reverse) can only be requested when we are in the paused state. Thecurrent frame on which we are paused is known from the MpegPause class.This frame is located in the GOP index of the trick file. Then the clipstart point and length are modified in the Clip instance with the trickfile position computed from the beginning of the clip. So, the Clipclass handle these trick files in a manner similar to the main file. Thecurrent logical block number is updated with the block address in thetrick file recomputed from the beginning of the main clip. In fact, aseek is performed in the trick file as it was part of the main file,which is totally transparent for the ClipList and Clip classes. Thetransition from fast forward to pause is handled in a similar fashion.The clip start and length and the logical block number are againupdated. The smooth transitions from pause to fast forward and from fastforward to pause are done in the same way as for regular play. There isa splicing from the pause stream to the play stream.

The class hierarchy for trick file handling is shown in FIG. 28. TheMpegFast, MpegFastForward and MpegFastReverse class handles the GOPgeneration from the initial file. This is the common procedure forbuilding the GOP whatever the source and the destination.RealTimeFastFwd and RealTimeFastRev are the class instantiated when areal time fast forward (reverse) has to be done. They manage thereal-time buffer flow to the player. There is a derivation of themethods takeBuffer and returnBuffer which uses the base class to buildthe GOP in the buffer to be played. The main file access is done using abuffer pool.

TrickFilesGenerate is the class instantiated to generate trick filesforward and reverse. It inherits from TrickFileAccess the methods forreading the original file some buffers and for writing the trick fileand its meta-data. It inherits from MpegFastForward the methods forbuilding the GOP and for managing the advance in the file.

The computation of the next I frame to play is done by MpegFast,MpegFastForward and RealTimeFastFwd. When a trick file generationcommand is invoked, a thread is created and started and the generationitself is done off-line. A call-back is sent to the video service whenthe generation is completed. The class TrickFilesGenerate generates thetrick file forward, and then, using the GOP index built in memory, theclass TrickFiles Generate generates the trick file reverse.

When there is a transition from play to pause, the only latency issue isrelated to the buffer queue handled by the player and to the GOP size.The stream can build immediately the active pause GOP, and then this GOPwill be sent at the end of the current GOP with a splicing between thesetwo streams.

When there are transitions from pause to regular play or fast forwardand fast reverse, a seek in the file is done. This means that thecurrent buffer pool content is invalidated and the buffer pool is filledagain. Play can start again while the buffer pool is not completelyfull, as soon as the first buffer is read. The buffer pool prefillingcan continue as a background process. The issue here is that there is arisk to generate an extra load on the cached disk array as well as onthe stream server side when the buffer pool is being prefilled.

To avoid too frequent transitions from play to fast forward and fastreverse, there is a limitation of the number of requests per second foreach stream. This limitation is part of the management of the videoaccess commands. A minimum delay between two commands is defined as aparameter. If the delay between a request and the previous one is toosmall, the request is delayed. If a new request is received during thisdelay, the new request replaces the waiting one. So the last receivedrequest is always executed.

The volume parameter (vsparams) file contains these new parameters forthe trick mode files:

TrickFileExtensionSize:<percent>:

DefaultFastAcceleration:<acceleration>:

DMtrickFileGen:<mask of reserved DM> (This parameter is a mask of thestream servers that can be chosen to perform the trick file generation.The default value is 0xfffc: all of the stream servers.)

DMtrickFileGenBW:<bandwidth used for trick file generation>(Thisparameter is the value of the bandwidth effectively used by the streamserver for the trick files generation.)

The video service routines are modified to operate upon the EMPG2 files,and in particular to compute the size of the EMPG2 files, to allocatethe volume for the main file and the trick files, and to generate thetrick files. The volume creation functions (VAPP) and volume accessfunctions (RRP) use the EMPEG2 files in the same way as MPEG2 files.This means that a MPEG2 volume is created on the stream server. BothMPEG2 and EMPEG2 files can be used in the same session or play-list. Thesession encoding type is MPEG2. In record (or copy-in), the number ofblocks allocated for an EMPEG2 file is computed using the percentage ofsize to add. At the end of record (or copy-in), the number of blocks isadjusted using the number of blocks returned by the stream server (byCMSPROC_GETATTR) and adding the percentage for trick files. The trickfiles validity and generation date are stored by the video service inthe asset structure. The bandwidth allocated to the TrickFilesGeneratecommand is defined in the volume parameters (vsparams or vssiteparams).The selection of a stream server to generate the trick files takes intoaccount this bandwidth only. If preferred stream servers are specifiedin vsparams (or vssiteparams), then the selected stream server will beone of these specified stream servers.

In a preferred implementation of the video service software, a newencoding type is created. The encoding type enum becomes:

enum encoding-t{ ENC_UNKNOWN = 0, /* unknown format */ ENC_RAW = 1, /*uninterpreted data */ ENC_MPEG1 = 2, /* constrained MPEG1 */ EMC_MPEG =3, /* generic MPEG */ ENC_EMPEG2 = 4, /* MPEG2 with trick filesextension */ };

The encoding information accessible by VCMP_EXTENDEDINFO includesinformation about trick files:

struct trickFilesInfo_t{ ulong_t generationDate; /* date/time of thegeneration of the trick files */ rate_factor_t acceleration; /*acceleration factor */ ulong_t framesNumber; /* frames number in eachtrick file (FWD and REV) */ ulong_t gopNumber; /* GOP number of eachfile */ }; struct EMPEG2info_t{ MPEG2info_t MPEG2info; trickFilesInfo_ttrickFiles<>; }; union encodingInfo_(‘3)t switch (encoding-t enc){ caseENC_MPEG: MPEG2info_t MPEG2info; case ENC_EMPEG2: EMPEG2info_tEMPEG2info; default: void; }; The video service software includes a newprocedure (VCMP_TRICKFILESGEN) for trick file generation, which uses thefollowing structures: struct VCMPtrickgenres_t{ VCMPstatus_t status;tHandle_t handle; }; struct VCMPtrickfilesargs_t{ name_t clipname;bool_t overwritelfExists; rate_factor_t acceleration; };VCMPtrickgenres_t VCMP_TRICKFILESGEN (VCMPtrickfilesargs_t) = 36,

If the trick files already exist and if the boolean overwriteIfExists istrue, then the trick files are generated again, in the other casenothing is done. Acceleration is the acceleration as defined and usedfor the controlled speed play function. It is a percentage of the normalspeed, it must be greater than 200 and smaller than 2000. The specialvalue of 0 can be used to generate files with the default accelerationdefined in vssiteparams. The procedure starts the generation process.The completion is notified by a callback.

The video service includes a new option to copyin and copyout. Theoption is added to allow a user to copy all the file or the main assetonly. For compatibility with old client programs, the following newprocedures are added:

VCMPcopyres_t  VCMP_FULL_COPYIN (copyinargs2_t) = 37, VCMPcopyres_t VCMP_FULL_COPYOUT (copyoutargs2_t) = 38,

These new procedures take the same interface as the existing one, butare used to copy-in the complete file: meta-data+Asset+trick files.

The video service includes a new procedure VCMP_TRICKFILESGENCOMPLETED,which uses the following structures:

struct VCMPtrickfilescomplete_t{ tHandle_t handle; VCMPstatus_t status;}; VCMPstatus_t TRICKFILESGENCOMPLETED (VCMPtrickfilescomplete_t) = 10,

The video service includes new procedures are added for handling trickmode generation arguments, which uses the following structures:

struct cms_trick_gen_args { Handle_t Vshandle; name_t name; bool_toverwriteIfExists; rate_factor_t acceleration; bandwidth_t reservedBw;}; cms_status CMSPROC_GEN_TRICK_FILES (cms_trick_gen_args) = 34, structtrick_gen_completed_args { Handle_t Vshandle; cms_status status; }; voidCTLPROC_TRICKGENCOMPLETED (trick_gen_completed_args) = 8,

The video service includes the following option to force theregeneration of trick files even if they exist:

nms_content—gentrick <name> [<-f>] [acceleration]Without this option, anerror code is returned if the trick files exist. “Acceleration” is anacceleration factor. If it is not present, the default value is taken invsparams.

The video services include a encoding information access function(nms_content-m). This function produces a displayed output containing,for each trick file generated, the acceleration, the generation date andtime, the frames number, and the GOP number.

For the use of an FTP copy function with the trick files, the followingnew commands are added:

nms_content—copyinfull <same arguments as—copyin>

nms_content—copyoutfull <same arguments as—copyout>

Another application of the SNR scaling of the invention is to reduce thebit rate of an MPEG-2 transport stream in order to allow combiningmultiple MPEG-2 transport streams to match a target bit rate for amultiple program transport stream. For example, FIG. 29 shows a systemfor combining an MPEG-2 audio-visual transport stream 411 with an MPEG-2closed-captioning transport stream 412 to produce a multiplexed MPEG-2transport stream 413. In this case, the closed captioning transportstream 412, containing alphanumeric characters and some control datainstead of audio-visual information, has a very low bit rate compared tothe audio-visual transport stream 411. Assuming that the target bit ratefor the multiplexed transport stream 413 is the same as the bit rate ofthe audio-visual transport stream 411, there need be only a slightdecrease in the bit rate of the audio-visual transport stream, and thisslight decrease can be obtained by occasionally removing one non-zero ACDCT coefficient per 8×8 block. Therefore, in the system of FIG. 29, theaudio-visual transport stream 411 is processed by a program module 414for selective elimination of non-zero AC DCT coefficients to slightlyreduce the average bit rate of this transport stream. A transport streammultiplexer 415 then combines the modified audio-visual transport streamwith the closed captioning transport stream 412 to produce themultiplexed MPEG-2 transport stream 413.

In order to determine whether or not any non-zero AC DCT coefficientshould be eliminated from a next 8×8 block in the audio-visual transportstream 411, a module 421 is executed periodically to compute a desiredbit rate change in the audio-visual transport stream 411. For example,respective bit rate monitors 416, 417 may measure the actual bit rate ofthe audio-visual transport stream 411 and the closed captioningtransport stream 412. Alternatively, if it is known precisely how thesetransport streams are generated, presumed values for the bit rates ofthese transport streams may be used in lieu of measured bit rates. Thecomputation of the desired bit rate change also includes the desired bitrate 418 for the multiplexed MPEG-2 transport stream, and a bit rate 419of multiplexer overhead, representing any net increase in bit raterelated to the multiplexing of the audio-visual transport stream 411with the closed captioning transport stream 412. An adder/subtractor 420combines the various bit rate values from the inputs 416, 417, 418, and419 to compute the desired bit rate change in the audio-visual transportstream 411. From the adder/subtractor 420, the module 421 converts thedesired change in bit rate to a desired number of bits to be removed percomputational cycle (e.g., per millisecond). This number of bits to beremoved per computational cycle is received in an adder/subtractor 422,and the output of the adder/subtractor is received in an integrator 423.A limiter 424 takes the sign (positive or negative) of the integratedvalue to produce a flag indicating whether or not one non-zero AC DCTcoefficient should be removed from the coefficients for the next 8×8block, assuming that the next block has at least one non-zero AC DCTcoefficient. (Alternatively, a non-zero AC DCT coefficient could beremoved only if the 8×8 block has more than a predetermined fraction ofthe average number of AC DCT coefficients per 8×8 block.) The particularnon-zero AC DCT coefficient to remove in each case can be selected usingany of the methods discussed above with reference to FIGS. 14, 15, orFIG. 20. For example, the coefficient to remove could be the lastnon-zero AC DCT coefficient in the scan order. Alternatively, thenon-zero AC DCT coefficient having the smallest magnitude could beremoved so long as its removal does not cause an escape sequence.

When the module 414 removes a non-zero AC DCT coefficient from a 8×8block, it sends the number of bits removed to the adder/subtractor 422.In a preferred implementation, the operations of the adder/subtractor422, integrator 423, and limiter 424 are performed by a subroutinehaving a variable representing the integrated value. During eachcomputational cycle, the variable is incremented by the number of bitsto be removed per computational interval, and whenever the module 414removes a non-zero AC DCT coefficient from a 8×8 block of theaudio-visual transport stream, the variable is decremented by the numberof bits removed.

Although the system in FIG. 29 has been described for achieving a slightreduction in bit rate of the MPEG-2 audio-visual transport stream 411for combining multiple transport stream to produce a muliplexed MPEG-2transport stream, it should be apparent that it could be used forobtaining relatively large reductions in bit rate. In this case, themodule 414 would use the procedure of FIGS. 14, 15 or preferably FIG.20, and a multi-level comparator 424 would be used instead of asingle-level comparator 424. The multi-level comparator would determinea desired number of non-zero coefficients to discard per 8×8 block basedon the value of the output of the integrator 423. The maximum number ofnon-zero AC coefficients to keep for each 8×8 block (i.e., the value ofthe parameter “k”), for example, would be determined by subtracting thenumber of non-zero AC DCT coefficients in the 8×8 block from the desirednumber to discard, and limiting this difference to no less than apredetermined fraction of the average number of non-zero AC coefficientsper 8×8 block.

In view of the above, there have been described methods of efficient SNRscaling of video originally present in a high-quality and nonscalableMPEG-2 transport stream. To reduce bandwidth of non-scalable MPEG-2coded video, certain non-zero AC DCT coefficients for the 8×8 blocks areremoved from the MPEG-2 coded video. In one implementation,high-frequency AC DCT coefficients are removed at the end of thecoefficient scan order. This method requires the least computation andis most desirable if the reduced-bandwidth MPEG-2 coded video is to bespatially sub-sampled. In another implementation, the smallest-magnitudeAC DCT coefficients are removed. This method may produce an undesirableincrease in the frequency of occurrence of escape sequences in the (run,level) coding. This frequency of escape sequences can be reduced byretaining certain non-zero AC DCT coefficients that are not the largestmagnitude coefficients and would otherwise be removed, and by increasingthe quantization scale factor to reduce the coefficient levels. Thereduced-bandwidth MPEG-2 coded video can be used for a variety ofapplications, such as browsing for search and play-list generation, bitstream scaling for splicing, bit-rate adjustment via video qualityalteration for services with limited resources, and bit rate control ofmultiplexed transport streams.

What is claimed is:
 1. A method of scaling non-scalable MPEG-2 codedvideo to produce reduced-bandwidth, reduced-quality MPEG-2 coded video,the non-scalable MPEG-2 coded video including a set of non-zero ACdiscrete cosine transform (DCT) coefficients for 8×8 blocks of thenon-scalable MPEG-2 coded video, said method including the step ofremoving non-zero AC DCT coefficients from the non-scalable MPEG-2 codedvideo so that there are no more than a selected number of non-zero ACDCT coefficients in the reduced-quality MPEG-2 coded video for each ofthe 8×8 blocks, which includes inspecting magnitudes of the non-zero ACDCT coefficients to sort at least some of the non-zero AC DCTcoefficients in each 8×8 block in terms of magnitude for retaining up tothe selected number of largest magnitude non-zero AC DCT coefficientsfor said each 8×8 block, and removing any additional non-zero AC DCTcoefficients for said each 8×8 block.
 2. The method as claimed in claim1, wherein the method includes, for each 8×8 block having a number ofnon-zero AC DCT coefficients greater than the selected number, executinga selected one of a plurality of procedures for finding the selectednumber of largest magnitude non-zero AC DCT coefficients for the 8×8block, the selected one of the plurality of procedures being selectedbased on the number of non-zero AC DCT coefficients that said each 8×8block has in the non-scalable MPEG-2 coded video.
 3. The method asclaimed in claim 2, which includes comparing the selected number to thenumber of non-zero AC DCT coefficients for said each 8×8 block to selectthe selected one of the plurality of procedures.
 4. The method asclaimed in claim 3, wherein at least one of the procedures sorts atleast the selected number of the non-zero AC DCT coefficients for the8×8 block.
 5. A method of scaling non-scalable MPEG-2 coded video toproduce reduced-bandwidth, reduced-quality MPEG-2 coded video, thenon-scalable MPEG-2 coded video including a set of non-zero AC discretecosine transform (DCT) coefficients for 8×8 blocks of the non-scalableMPEG-2 coded video, said method including the step of removing non-zeroAC DCT coefficients from the non-scalable MPEG-2 coded video so thatthere are no more than a selected number of non-zero AC DCT coefficientsin the reduced-quality MPEG-2 coded video for each of the 8×8 blocks,which includes inspecting magnitudes of the non-zero AC DCT coefficientsto order at least some of the non-zero AC DCT coefficients in each 8×8block in terms of magnitude for retaining up to the selected number ofthe non-zero AC DCT coefficients for said each 8×8 block that differ inmagnitude from up to the selected number of largest magnitude non-zeroAC DCT coefficients for said each 8×8 block by no more than a certainlimit.
 6. The method as claimed in claim 5, wherein the ordering of atleast some of the non-zero AC DCT coefficients in terms of magnitudeincludes classifying the non-zero AC DCT coefficients for each 8×8 blockinto groups having magnitudes that are substantially the same.
 7. Themethod as claimed in claim 6, wherein the ordering of at least some ofthe non-zero AC DCT coefficients in each 8×8 block in terms of magnitudeincludes performing a hashing operation upon magnitudes of the non-zeroAC DCT coefficients for said each 8×8 block.
 8. The method as claimed inclaim 6, which includes maintaining a list of the non-zero AC DCTcoefficients for each group, the non-zero AC DCT coefficients in eachlist being ordered by a scan order of the AC DCT coefficients in thenon-scalable MPEG-2 coded video, the non-zero AC DCT coefficients thatare retained in the reduced-quality MPEG-2 coded video being read fromthe lists in the scan order.
 9. A method of scaling non-scalable MPEG-2coded video to produce reduced-bandwidth, reduced-quality MPEG-2 codedvideo, the non-scalable MPEG-2 coded video including a set of non-zeroAC discrete cosine transform (DCT) coefficients for 8×8 blocks of thenon-scalable MPEG-2 coded video, said method including the step ofremoving non-zero AC DCT coefficients from the non-scalable MPEG-2 codedvideo so that there are no more than a selected number of non-zero ACDCT coefficients in the reduced-quality MPEG-2 coded video for each ofthe 8×8 blocks, and which further includes selecting an initial set ofthe non-zero AC DCT coefficients to include in the reduced-qualityMPEG-2 coded video, (run, level) coding the initial set of the non-zeroAC DCT coefficients, and upon finding an escape sequence in the (run,level) coding of the initial set of the non-zero AC DCT coefficients,searching for a non-zero AC DCT coefficient of the non-scalable MPEG-2coded video that is not in the initial set of the non-zero AC DCTcoefficients and that would eliminate the escape sequence when includedin the reduced-quality MPEG-2 coded video, and upon finding such anon-zero AC DCT coefficient of the non-scalable MPEG-2 coded video,including such a non-zero AC DCT coefficient in the reduced-qualityMPEG-2 coded video.
 10. The method as claimed in claim 9, wherein theinitial set of the non-zero AC DCT coefficients includes, for each 8×8block, up to a certain number of non-zero AC DCT coefficients thatdiffer in magnitude from up to the certain number of largest magnitudenon-zero AC DCT coefficients by no more than a certain limit.
 11. Themethod as claimed in claim 10, wherein the initial set of the non-zeroAC DCT coefficients includes, for each 8×8 block, up to the certainnumber of largest magnitude non-zero AC DCT coefficients.
 12. The methodas claimed in claim 9, which further includes increasing a quantizationscale for the reduced-quality MPEG-2 coded video in order to reducelevels of the non-zero AC coefficients retained in the reduced-qualityMPEG-2 coded video.
 13. The method as claimed in claim 9, which includesadjusting a quantization scale for the reduced-quality MPEG-2 codedvideo in order to achieve a desired frequency of escape sequences in thereduced-quality MPEG-2 coded video.
 14. The method as claimed in claim13, which includes adjusting the selected number to obtain a desiredaverage bit rate for the reduced-quality MPEG-2 coded video.
 15. Themethod as claimed in claim 9, wherein the non-zero AC DCT coefficientsare removed from the non-scalable MPEG-2 coded video in order to obtaina reduction in bit rate of a first MPEG2-transport stream containing theMPEG-2 coded video for combining the first MPEG-2 transport stream withat least a second MPEG-2 transport stream to produce a multiplexedtransport stream having a desired bit rate.
 16. A method of scalingnon-scalable MPEG-2 coded video to produce reduced-bandwidth,reduced-quality MPEG-2 coded video, the non-scalable MPEG-2 coded videoincluding a set of non-zero AC discrete cosine transform (DCT)coefficients for 8×8 blocks of the non-scalable MPEG-2 coded video, saidmethod including the step of removing non-zero AC DCT coefficients fromthe non-scalable MPEG-2 coded video by selecting an initial set of thenon-zero AC DCT coefficients to include in the reduced-quality MPEG-2coded video, (run, level) coding the initial set of the non-zero AC DCTcoefficients, and upon finding an escape sequence in the (run, level)coding of the initial set of the non-zero AC DCT coefficients, searchingfor a non-zero AC DCT coefficient of the non-scalable MPEG-2 coded videothat is not in the initial set of the non-zero AC DCT coefficients andthat would eliminate the escape sequence when included in thereduced-quality MPEG-2 coded video, and upon finding such a non-zero ACDCT coefficient of the non-scalable MPEG-2 coded video, including such anon-zero AC DCT coefficient in the reduced-quality MPEG-2 coded video.17. The method as claimed in claim 16, wherein the initial set of thenon-zero AC DCT coefficients includes, for each 8×8 block, up to acertain number of non-zero AC DCT coefficients that differ in magnitudefrom up to the certain number of largest magnitude non-zero AC DCTcoefficients by no more than a certain limit.
 18. The method as claimedin claim 17, wherein the initial set of the non-zero AC DCT coefficientsincludes, for each 8×8 block, up to the certain number of largestmagnitude non-zero AC DCT coefficients.
 19. The method as claimed inclaim 16, which further includes increasing a quantization scale for thereduced-quality MPEG-2 coded video in order to reduce levels of thenon-zero AC coefficients retained in the reduced-quality MPEG-2 codedvideo.
 20. The method as claimed in claim 16, which includes adjusting aquantization scale for the reduced-quality MPEG-2 coded video in orderto achieve a desired frequency of escape sequences in thereduced-quality MPEG-2 coded video.
 21. The method as claimed in claim20, which includes precluding each of the 8×8 blocks in thereduced-quality MPEG-2 coded video from having more than a selectednumber of non-zero AC DCT coefficients, wherein the selected number isadjusted to obtain a desired average bit rate for the reduced-qualityMPEG-2 coded video.